Coin-based homunculi known as the Greeed have awoken from an 800-year slumber! The only hope of stopping them in their pursuit of human desires is a rogue Greeed named Ankh, reduced to the form of a disembodied arm, and the wandering hermit Eiji Hino.
Presented with a belt and three Core Medals, Eiji steps forward as the champion of human desire - Kamen Rider OOO!
A mysterious arm reawakens the Greeed from their slumber and decides to use Eiji Hino, a cheerful wanderer, as his means of getting back what he desires as Kamen Rider OOO.
Ankh explains the Greeed and the purpose of the Medals to Eiji, even as the human desires to merely use his new power to fight for good instead of for selfish needs.
Ankh becomes troubled when Eiji prevents him from eliminating his possessed body's sister while another Greeed uses a man's gluttony to create more Medals.
Eiji gets to the root of the Komoris' problem involving greed and loss of ambition while Ankh fights for survival after a failed confrontation with Uva.
A serial bomber tied to a nest of Yummies leads to Eiji to reveal his past to Hina and forces him to encounter a strange man he believes is Mezool's victim.
Eiji must figure the means of stopping the mad bomber and his Yummies to prevent any more lost lives, even with Maki observing the creation of his worker's monsters.
Eiji teaches a lesson to Tsukuba and Gotou about going forth to gain what is desired; while Ankh tries to figure a way to manage the flying Ageha Yummy using Gamel.
Ankh is at a breaking point with everything surrounding his human host; a highly sought after doctor has been repeatedly removed from the operating room, making her frustration a perfect target for the Greeed.
Both Hina's brother and the hospital director are in critical condition after Ankh's detachment and the Yummy attack. Eiji must find both Ankh and the affected surgeon before it's too late.
Ankh finds one of his core medals; Gotou is transporting a truck full of medals to Doctor Maki; Kazari appears to Ankh and Eiji, telling them that Hina is in Gotou's truck.
The Greeed take an interest in a girl who wants to get better at kendo; Ankh takes an interest in Kamen Rider Birth; Hina gives both Eiji and Ankh gifts for saving her.
Kougami tries to give Eiji one of Ankh's medals, but he refuses; a face from the detective's past shows up at Cous Coussier; thanks to Kazari, a criminal that the detective put away is on the streets and looking for revenge.
Ankh tricks Kazari by telling him that Eiji knows where his last medal is and then chases after Hina who has his own medal; Date and Gotou discuss the skills need to be Kamen Rider Birth; Kougami changes his mind and sends Ankh's medal to Eiji.
The Cous Coussier gang runs into a young boy getting attacked for telling off a man for littering, learning that it's something he picked up from his father; the Greeed creates a Yummy from the desire to help people.
Susumu lets his desire for justice get out of hand and it's up to Eiji to stop him before it's too late; Gotou and Takashi have a heart-to-heart conversation about their ideals.
Eiji is still super beat up from the previous battle but he is the only one who believes that Ankh did not create the bird Yummy; Kazuki heads back to the ring as Date and Gotou rush to stop him from injuring himself more.
Kougami invites Eiji and Ankh over to celebrate the 1000th episode of Kamen Rider and decides to make a movie to celebrate; Kazari is drawn to a Shocker grunt who joins the movie.
Dr. Maki and Chiyoko have a heart-to-heart talk about his sister; Kazari takes Ankh's memoryless body under his wing; Kougami tells Gotou the story of the medals as Eiji and Date get Ankh to finally tell his story.
Dr. Maki and Kazari try to awaken a brand new Greeed; a man has been unknowingly using Greeed money to pay off an old debt; Eiji gets infected by purple core medals.
An x-ray reveals that there should be nothing wrong with Eiji; Date tells the others about the time he previously ran into Eiji; Eiji uses the purple medals.
Eiji is forced to use the purple medals again; an old friend from High School invites Eiji to an amusement park where Hina is then kidnapped; a Yummy abducts Date and Gotou.
Hina wins first place in a fashion contest and is offered a chance to study in France; Dr. Maki creates a Yummy that destroys people's dreams; Date collapses in battle and Kougami finds an interesting x-ray on his brain.
Hina's brother has woken up, leaving Ankh without a host; Eiji discovers Hina destroying all of her clothes after getting her dream broken; Uva revives Mezool and Gamel.
Ankh has nightmares about losing Eiji to the purple medals and other Ankh; a man has set up surveillance cameras all over the neighborhood, trying to force out Eiji and Ankh because they make the neighborhood look bad; now that Gotou is Birth, Kougami assigns Satonaka as Birth's new partner.
The Cous Coussier crew get thrown out of town where they decide to throw Ankh the birthday party for Hina's brother; the Yummy has holed himself up with a human barricade, forcing our heroes to wait until he surfaces again; the other Ankh waits for his other self to let his guard down so that he can become whole again.
Ankh is absorbed into a single body, but the two halves are still not completely whole; Eiji begins to wonder about his place in the Izumi family's life as Shingo tries to help him out in Ankh's place; Gotou helps Eiji on his mission to rescue Ankh, but Eiji is stopped by Dr. Maki.
Dr. Maki explains to Eiji about the process of becoming a Greeed and the dulling of the human senses, leading Eiji to discover that it is already happening to him; a Yummy is trapping people in ice and calling out for OOO's help, but HIna refuses to let him go; Eiji does battle with Ankh, destroying.his core medals.
After Ankh's cores were destroyed, he becomes in need of a body and yet again takes the detective's, also realizing that he's different now and creates a Yummy; Hina comes to the conclusion that she's been too selfish and reliant in regards to other people, making the decision to change and stand by Eiji no matter what; Ankh revives Uva and makes a deal with Dr. Maki to become his container of medals.
Kazari and Ankh butt heads over who is the leader of the Greeed; Hina and Gotou try to find Eiji's desire so that the purple medals will stop making him go berserk; Hina stops Ankh from taking medals from Eiji, but Kazari swoops in and gets the last of his nine medals; Eiji destroys one of Kazari's core medals, allowing Dr. Maki to steal the remaining medals from a weakened Kazari.
With Birth in need of repair, Erika retrieves the Prototype Birth driver in order to make do; because they are at a major disadvantage, Eiji and Gotou decide to go on the offense and attack the Greeed first; the other Greeed confront Dr. Maki and Ankh, with Mezool fully reviving and leaving Gamel, who also has the remainder of his medals.
Gamel rampages throughout the city trying to revive Mezool with one of her core medals; Eiji washes up on the beach with the purple medals acting up, and before he can find his driver, Ankh shows up to stop him from turning into a Greeed; Gotou has his Birth driver back, but he's in trouble and Erika gives the prototype to a familiar face.
Dr. Maki makes Eiji go berserk and fight Ankh when he won't give up the purple medals, but Ankh can't force himself to purposefully hurt Eiji; Ankh believes that one of his medals was broken by Dr. Maki and lets Hina know that her brother will be returned soon; Eiji discovers his true desire; Uva is fully revived.
Eiji receives a present from Kougami; Dr. Maki decides to make Uva the container for the core medals; Ankh returns to Eiji's side as they prepare for the final battle.
