Many of parallel worlds were merging to bring a great catastrophe; Kamen Rider Decade travels around the worlds to prevent it with exploiting all the other Kamen Rider’s skills.
After Natsumi Hikari has a nightmare of every Kamen Rider being defeated by a figure called Decade, she helps her grandfather Eijiro deal with unsatisfied customers. The photographer, Tsukasa Kadoya, seems to be at fault, but he simply tells Natsumi the photos are bad because he does not belong in this world. After the argument, various holes in reality open, allowing the Orphnoch, Undead, Makamou, Worms, Imagin, and Fangires to start terrorizing their world. In the bedlam, she finds a corroded belt and book, recalling it as Decade's from her dream. She gives Tsukasa the belt and book, allowing him to transform into Kamen Rider Decade by using a card from the book and inserting it into the belt.
Arriving in the World of Kuuga, Tsukasa takes the guise of a policeman and watches Kuuga destroy a pair of Grongi, Ra-Dorudo-Gu and Me-Gyarido-Gi, before reverting to his human form, Yusuke Onodera. Tsukasa later destroys Go-Baberu-Da as Decade, but is mistaken for a Grongi by Yusuke and Ai. When Yusuke and Ai arrive at the Hikari Studio to discuss this strange Grongi, Tsukasa offers Ai his help in solving the mystery behind the Grongi's female targets and the location of their lair. On the foot of Mount Hitoki, the Grongi's Lair, Tsukasa reveals he used Ai as bait to lure out the remaining Grongi, revealing their plan to revive their leader before ruining their game. While attempting to destroy the Grongi, Decade is attacked by Kuuga, who reveals he knows about Decade's mission and will stop him at all costs.
After fending off the Hoppers who disappeared into another dimensional rift, Yusuke reveals he was told of Decade by a mysterious man. When Ai and her task force go to investigate Mount Hitoki, a black mist begins to rise from within, signaling the revival of N-Gamio-Zeda. Resurrected by the dimensional chaos, he uses his miasma to convert humans into Grongi. While Yusuke saves Ai and takes her to the hospital, Tsukasa fights the Grongi alone. A dying Ai requests Yusuke to overcome his angst and fight to protect everyone's smiles, and so Kuuga and Decade work together to destroy the remaining Grongi. N-Gamio-Zeda, in a last effort, absorbs all the Grongi to power himself up, but he is ultimately overpowered by both Decade and Kuuga, who destroy him with a combo attack.
Arriving in the World of Kiva, Tsukasa and company discover that the Fangires of this world have made a truce with the humans. However, when Tsukasa finds the Lion Fangire attacking a young woman, he transforms into Decade to fight him. Wataru, the King of the Fangire Race, witnesses this and becomes Kamen Rider Kiva to fight him with the aid of his Arms Monsters, forcing Decade to assume Kamen Rider Kuuga's Mighty, Titan, and Dragon Forms to counter. Yusuke manages to stop the fight, and the Swallowtail Fangire reveals that the young woman was actually a renegade Fangire. Yusuke reveals he has promised to help Wataru because he is uncomfortable with being the King.
As Tsukasa is fighting against Kaixa in another dimension, the Beetle Fangire defeats Wataru and becomes the new Fangire king, claiming Kivat. After the new King dissolves the truce, Yusuke arrives to protect Wataru before the boy runs off. After Yusuke expresses his reasons for helping the dethroned boy, Wataru takes refuge in the Hikari Studio. After being caught up on current events, Tsukasa makes his way to Castle Doran to save Yusuke as the Beetle Fangire absorbs the Arms Monsters into himself. After destroying the Swallowtail Fangire, Decade encounters the Beetle Fangire, who assumes Kiva's DoGaBaKi Form. When their fight makes its way to Yusuke and Wataru, the boy finds his courage to overcome his Fangire impulses and declares himself the King once more.
Upon arriving in the World of Ryuki, Natsumi goes to the Atashi Journal headquarters to learn about the world's Kamen Riders from its editor, Reiko Momoi. After Natsumi's arrival at the journal's offices, Reiko is mysteriously murdered, leaving Natsumi as the culprit as she had a small fork in her hand. Natsumi is arrested under suspicion of murder, and is to be sentenced with a Kamen Rider Trial: a system where Kamen Riders fight in the Mirror World to determine the outcome of criminal cases. Natsumi calls upon Tsukasa to be her lawyer, who meets with Shinji Tatsumi to hear what he knows of what happened to Reiko. As Shinji reveals that he is Kamen Rider Ryuki and he believes Natsumi was framed, his former friend Kamen Rider Knight is fighting in the Kamen Rider Trial, defeating several opponents. Shinji suggests to Tsukasa that he should fight as Decade in the Kamen Rider Trial to clear Natsumi's name.
After defeating Knight, Decade is about to finish him off when Ryuki pleads with him not to deliver the final blow, as he feels Knight must answer for his crime by his hand. However, Abyss uses this time to make his move to take out all three Riders at once, forcing them to retreat with Ren refusing to answer Shinji as he leaves. After her brief encounter with Narutaki, Natsumi learns that Shinji and Tsukasa are convinced that Ren is Reiko's murderer, although Yusuke sees otherwise. Meanwhile, a wounded Ren battles Kamen Rider Odin and defeats him, emerging from Mirror World with Odin's Time Vent as Shinji and Yusuke find him. Learning that Ren obtained the card to save Reiko, Shinji uses the card to go back in time as Tsukasa holds Abyss off. Arriving seconds before Reiko's death, Shinji and Tsukasa manage to save Reiko with the latter cornering her true murderer, Kamata.
Arriving in the World of Blade, Tsukasa assumes the disguise of the Chief Waiter of the BOARD Corporation, an organization whose agents become Kamen Riders to battle the Undead. When the Elephant Undead goes on the attack, Sakuya Hishigata battles the Undead as Kamen Rider Garren as Kazuma Kendate arrives and transforms into Kamen Rider Blade. When the Capricorn Undead joins the fray, Tsukasa joins the fight, dispatching the Elephant Undead using Ryuki's powers. Afterwards, Natsumi and Yusuke meet BOARD's president Hajime Shijo, who decides to promote Tsukasa to the rank of King while Kazuma Kendate is demoted from Ace to 7 and Mutsuki Kuroba is promoted from King to Ace, thereby becoming Kamen Rider Leangle.
Decade continues his fight with Chalice, with Garren losing his Garren Buckle in the process. The fight comes to a sudden halt when Kamen Rider Todoroki shows up. Back at BOARD, Hishigata is demoted to Rank 3 while Kazuma is fired from BOARD. After he understands Tsukasa's intent to help him, Kazuma stays on in the kitchen as he opens the Ace Lunch to all BOARD members. Meanwhile, Mutsuki and Hishigata search for Chalice to get back the Rider systems, and are subsequently ambushed by Chalice, who knocks them out after they find out his true identity. After hearing that the two have supposedly resigned from the company, Kazuma decides to find Hajime with Yusuke's help.
Upon arriving in the World of Faiz and once more encountering the mysterious Daiki Kaito, Tsukasa finds himself in a school uniform and enrolled at Smart Brain High School. He learns that the high school is attacked on a nightly basis by beings called the Orphnoch, with only Faiz to protect the school. Daiki challenges Tsukasa to find out who Faiz is, and Tsukasa then challenges the Lucky Clover clique to a tennis match to see if any of them are Faiz. He beats them, learning that they are Orphnoch who are also after Faiz, and is forced to become Decade, using Blade's power against them. However, Decade is outclassed when Momose appears as the Tiger Orphnoch and warns him to back off.
After Tsukasa recovers the Faiz Gear, he learns Diend's reasons for wanting the belt before Takumi regains it so he and a shocked Yuri can escape on the Autovajin. However, as he exposed himself as the Wolf Orphnoch to protect her only to be forsaken, a saddened Takumi attempts to dispose of the Faiz Gear into the river. Managing to save the Gear while following him, Tsukasa returns to the photo studio where he put Yuri's confusion at ease. The next day, Daiki attacks Takumi to get the belt from him, nearly killing him when Tsukasa arrives with it. Refusing to take Tsukasa's words of something more precious than the Faiz Gear seriously, Daiki takes Decade's Ride Booker, and Narutaki takes advantage of the situation as he takes Tsukasa into another dimension to be killed by Kamen Rider Ryuga.
Upon arriving in the World of Agito, Yusuke is shocked that there are Grongi in the world which are being dealt with by Kamen Rider G3-X. He is further shocked when he sees an interview with Toko Yashiro, who is physically identical to the Ai Yashiro that died back in the World of Kuuga. Arriving at the police station, Yusuke offers to be the new user of the G3-X suit, but loses the position to Kaito. While this occurs, finding himself in a mailman outfit with a letter to Shouichi Ashikawa, Tsukasa and Natsumi find the man before they are attacked by what they think to be a Grongi. But during the fight, Tsukasa realizes he is fighting an Unknown, an Ant Lord called Formica Pedes.
After Yusuke saves Tsukasa from Delta's attack, Gills is about to fight when he's overcome with pain as the Unknown shows up to kill him. As Shouichi escapes, Toko recognizes him via G3-X's camera. While Tsukasa insists on protecting an annoyed Shouichi, Yusuke reports the defeat of the Grongi and the emergence of the Unknown. He finds Toko saddened, but has little time to help her as Kaito takes the G4 chip stored within his locker. Yusuke demands the chip back, but during the struggle, Toko blasts the chip to bits and a disappointed Kaito leaves. The next day, as Tsukasa continues to keep Shouichi safe from the Unknowns' attack, Yusuke learns that Shoichi was the original user of the G3 suit when he receives the letter for Shouichi from Toko.
When the Hikari Studio appears in the World of Den-O, Tsukasa finds himself dressed in a strange detective-like attire with possession of a Rider Pass. He is then suddenly possessed by an Imagin, who battles a Mole Imagin as Kamen Rider Den-O. After taking out the Imagin, the possessed Tsukasa walks off until Natsumi uses her pressure point move to drive the Imagin out of him. When the Imagin known as Momotaros tries to possess Tsukasa, again, he throws Yusuke in front of him and Yusuke becomes possessed. After easily overcoming M-Yusuke and treating him to pudding for answers, Tsukasa learns that the Imagin lost his physical form some time before they arrived.
After getting on the DenLiner, Tsukasa takes pictures of the DenLiner's gang before the Taros. But after Kohana sets the three Imagin in their place, they suddenty find the Hikari Studio as the front car. Kohana explains to Natsumi about the Imagin's new attack pattern as well as Momotaros, who Tsukasa decides to find. After surviving his confrontation with Diend and taking out another Mole Imagin, M-Yusuke tends to his injuries as Tsukasa finds him and scolds him for being an idiot. M-Yusuke senses the Imagin boss, and he and Tsukasa chase after the Alligator Imagin until he reveals himself. Using the Mole Imagin to cover his escape into the past, the Alligator Imagin battles Den-O and Decade, using the former's own DenGasher against him.
After helping the DenLiner gang in the past, Tsukasa and company begin their venture in the World of Kabuto. Posing as a ZECT Trooper, Tsukasa is deployed to aid Bright Troopers in the containment of a Worm as Kamen Riders Gatack and TheBee manage to exterminate it with the unseen aid of Kabuto. Soon after, Tsukasa brings Arata to the Studio to explain about his world, only to find a Worm had mimicked Tsukasa's form. After Natsumi uses the pressure point to expose one of the Tsukasas as a Worm, Decade fights it before it attacks a girl and is destroyed by Kabuto. Deciding to investigate the girl Mayu, they host a photo shoot for her, and Natsumi learns of Mayu's older brother who was supposedly killed by Kabuto. At ZECT's headquarters, TheBee's user, Sou Otogiri, tells Arata that the final prepation for the plans to capture Kabuto are nearly done.
Believing that TheBee's user is her brother, Mayu attempts to look for him with Natsumi's aid until they are attacked by a Worm. Counting on Kabuto to save them, Diend fights him to acquire his Clock Up system, summoning Ixa and Psyga to hold him down before Kabuto defeats them and runs off. By the time Tsukasa finds Natsumi and Mayu, the latter is shocked that she became a Worm as she runs off. She runs into Sou who reveals himself as the Phylloxera Worm and brings her to ZECT HQ as part of the Kabuto Capture plan, much to Arata's dismay before he is beaten to a bloody pulp questioning the plan. After escaping, Arata makes his way to Tsukasa and reveals the plan to him, confirming Tsukasa's notion. Kabuto falls for the trap as the Clock Down takes effect while the Worms reveal themselves among the ZECTroopers, with Sou exposing Kabuto as Mayu's real brother Souji.
Arriving in the last world, the World of Hibiki, Tsukasa and company begin the search for Kamen Rider Hibiki when they are ambushed by the Makamou Kappa. An Oni in training named Asumu battles the monster until Decade uses Kabuto's powers to destroy it. On learning he is Hibiki's pupil, Tsukasa has Asumu bring him to his mentor. Upon seeing Tsukasa, Hibiki, who is shirking off from his duties as an Oni, believes him to be the legendary Ongekidō Grand Master. Meanwhile, Zanki slays a band of Bakeneko as Ibuki arrives to take them out as well. But when the last Bakeneko escapes them, Decade destroys it with the two groups as witness. Tsukasa visits Ibuki's school afterwards and talks with Ibuki and Akira about three special scrolls. Meanwhile, after failing to rob Hibiki as he takes his leave, Daiki is forced into becoming Asumu's mentor.
Using Ryuki's powers, Decade battles Gyuki while Ibuki and Zanki argue whose fault this was. When Akira and Todoroki join the fray in their Oni forms to save Asumu as he tries to protect them, with Decade joining them by using Den-O's Ax Form, they manage to defeat Gyuki. He runs off before regressing to Hibiki, and Yusuke and Natsumi find him shortly after. Later that night, after talking to Asumu, Akira and Todoroki confront their mentors to put aside their differences, with Tsukasa supporting them. Ibuki and Zanki promote them to the role of new teachers of their respective schools. Meanwhile, Hibiki reveals the nature of Gyuki to Yusuke and Natsumi as Daiki arrives to claim the last scroll.
With their tour of the nine Kamen Rider Worlds complete, the Hikari Studio group returns to their home world, finding it as it was before the monsters appeared. Tsukasa is the only one who feels something is not right and questions the reason for the journey. Before Yusuke or Natsumi can attempt to answer, they encounter the violinist Otoya Kurenai, who thanks them for saving the world and tells Tsukasa that he earned a world of his own for his journey. As Natsumi goes out to see her TG Club friends and teacher for a high school reunion, Tsukasa and Yusuke have a short encounter with Narutaki who congratulates him on getting his own world.
After Tsukasa suddenly loses his powers, he learns from Kaito that he is actually in the World of Negatives. He later is seen by a talent agent and is offered to be signed onto his agency to become an idol. Although Yusuke acts as the voice of reason, he gets caught up in the fame seeking as well. Meanwhile, after her school friends return while waiting for Chinatsu, Natsumi that suggests that they should dig up the "treasure" they buried together, only to find that a parking lot was built over it and learning that Chinatsu moved their treasure prior. Once, Tsukasa finds Natsumi, he attempts to tell her that they are not in the real world, but Natsumi refuses to accept this as she runs off to the TG Club's hideout where she also meets the World of Negatives's Natsumi.
Arriving in Kaito's homeworld, the World of Diend, Tsukasa finds the world in peace and harmony, but with Kaito a wanted man. As Tsukasa assumes the role of a traveling salesman, he meets Kamen Riders Larc & Lance as they battle a Darkroach. After destroying the monster, Tsukasa learns that the world's residents kill Kamen Riders. As Tsukasa and company meet the head of area management committee, Junichi Kaito, Larc and Lance battle the Darkroaches until they receive aid from Diend. Saving them from reinforcements, Tsukasa learns from Haruka and Shin that the world is ruled by the mysterious Fourteen who uses his Roaches to manipulate humans.
Nearly killed by Lance, Kaito is spared when Larc stops her partner as he reminds her that he is the reason that Junichi is under Fourteen's control. After escaping the angry mob, parting from Haruka and Shin, Kaito and Tsukasa find Natsumi who tells them that Yusuke has been kidnapped by the Roaches before Junichi arrives. After remembering the events leading to his brother's enslavement, Kaito is reluctant to fight as he and Tsukasa transform to follow Junichi, leaving the brain-washed Yusuke behind and taking Natsumi to undergo the process as well.
Arriving in the new world with a strange card in hand, Tsukasa finds himself in the costume of a kuroko before running into Daiki who just stole a squid-like artifact. Daiki mentions that this world is not like the other A.R. Worlds before running off prior to the appearance of mysterious red fish-like monsters. Tsukasa is about to transform before several other Kuroko show up along with five young men and women who transform into the Shinkengers to fight drive them back into the Sanzu River.
After Tsukasa reveals himself, Takeru realizes that Tsukasa is the "Decade" Narutaki warned him about. Before the two can fight, Narutaki arrives to tell Tsukasa he is to blame for the terror brought unto the world by the Ayakashi Chinomanako who now has the powers of Diend, and that the only way to save the worlds he entered if he ceases to exist. After Takeru takes out the Nanashi and drives off Chinomanako as Shinken Red, Genta attacks Tsukasa believing that he is in league with Daiki until Natsumi takes him to the Hikari Studio, where Hikoma Kusakabe is hiding.
Tsukasa finds himself wearing strange clothes with another strange card as he, Natsumi, and Yusuke find themselves attacked by the Strange Demon Robot Schwarian and his Chaps as Kamen Rider Black RX battles them, with both sides believing Tsukasa to be Joe the Haze. When the Seamoon Fangire and Scorpion Imagin suddenly appear, Tsukasa becomes Decade to help RX out against the monsters with Agito's power as Schwarian runs off. After Natsumi stops RX from attacking Decade, RX takes her words to heart and sees Tsukasa as an ally.
Following Apollo Geist into another A.R. World, Decade is attacked by Kamen Rider Black until Diend arrives and summons Femme to hold him off. The next day, after learning that he is in the World of Black and seeing that he is on his own in saving Natsumi, Decade ambushes Guy as Ryuki and fights him until he uses a little girl as his hostage. When Decade takes a hit from Diend to save the child, Black watches and has second thoughts about him as Apollo Geist escapes.
Arriving in a new A.R. World, finding himself in a baseball uniform, Tsukasa notices the people are acting strange while saluting each other before he, Natsumi and Yusuke are singled out as rebels against Dai-Shocker with Propheta Cruentis, Go-Jaraji-Da, and Camponotus Worm Maxilla appearing to exterminate them. Amazon arrives to their aid, fighting the monsters with Decade backing him up as the people back up the surviving monsters under a boy named Masahiko.
Evading Apollo Geist and Llumu Qhimil with Diend's attempt to get the GaGa Armlet, Tsukasa has his wounds tended to by Natsumi in Dai-Shocker's school before Yusuke and a weakened Amazon join them, revealing that Masahiko took the GiGi Armlet as Ritsuko arrives and calls in the Dai-Shocker Soldiers after them despite Tsukasa questioning her faith in Dai-Shocker. Later after finding the Hikari Photo Studios in ruins, Amazon ponders if he should join them.
When Natsumi realizes that this is the world from her nightmares, she becomes wary for Tsukasa's well-being. Finding him in a tuxedo and feeling that this is the final chapter to their journey, they witness Wataru and Kazuma leading their respective Rider/monster groups against each other. After Rising Ixa and Leangle are killed off in the fight, Tsukasa makes his way to Wataru as he grieves over Ixa's death. Meeting the Fangire Queen Yuki, they learn from Wataru that the Riders are fighting each other to maintain the existence of their worlds.
After fighting off the Riders, Super Apollo Geist takes Natsumi to be his new bride as he flees. Kazuma Kenzaki appears, saying that Tsukasa's existence is the real cause of the worlds blending before confronting him as Kamen Rider Blade in King Form in order to force him to leave the world at once with Asumu keeping Yusuke and Wataru from interfering as Decade is easily defeated.
