Kamen Rider × Kamen Rider OOO & W Featuring Skull: Movie War Core

While fighting the male Pteranodon Yummy, Kamen Rider W receives aid from Ryu Terui, who is forced to fight without his Rider equipment due to his bride-to-be Akiko Narumi, who wants her friends to focus on her wedding. After the Yummy escapes, a tearful Akiko threatens to call off the wedding, but reveals she is upset because her deceased father, Sokichi Narumi, is unable to attend. Demanding to know why he became a Kamen Rider, the Pteranodon Yummy returns and uses the Memory Gaia Memory on her.

She finds herself witnessing a memory from Sokichi's past wherein he was working a case involving opera singer, Melissa, and an inhuman stalker. While watching Melissa's latest performance, Sokichi meets her seedy manager Kozo Yaguchi before Melissa's stalker, the Spider Dopant, attacks. Sokichi fights the criminal before the latter disappears with Gaia Memory dealer Eren Komori. Despite his partner Seiichiro "Matsu" Matsui's objections, Sokichi continues to investigate Melissa. He meets with his benefactor, Shroud, who tells him the Museum crime syndicate are moving forward with their master plan. While meeting with Matsu, Sokichi learns that Yaguchi's previous female clients have gone missing.

Receiving help from an informant named Stone, Sokichi breaks into the Yaguchi Talent Agency, where he finds the missing women are being used as test subjects for Gaia Memory research before being forced to watch Komori transform into the Bat Dopant and kill Stone. She and the Spider Dopant overwhelm Sokichi, but Shroud gives him the means to transform into Kamen Rider Skull. He defeats Komori's grunts, but she uses her powers to pin him down. After receiving a call from a young Akiko and promising to attend her future wedding, a reinvigorated Sokichi escapes Komori's grasp and pursues her and the Spider Dopant, only to find Yaguchi, who reveals he was only the Museum's Gaia Memory supplier before he is killed by a Spider Bomb concealed in his wife.

The next day, Sokichi deduces Matsu is the Spider Dopant and confronts him. Exposed, Matsu tries to defend himself until Melissa reveals Matsu took the Spider Gaia Memory to protect her from Yaguchi and that it warped his mind. Having previously implanted her with a Spider Bomb that will explode if she touches the person she loves and he fails to activate it, Matsu goes mad and kidnaps Melissa. After killing Komori when she tries to impede him, Sokichi pursues and eventually kills Matsu.

As Akiko emerges from the memory, the Pteranodon Yummy escapes once more.

The Kougami Foundation finds Oda Nobunaga's mummified remains and use it to create a Cell Medal-based homunculus, which soon escapes and transforms into a monster. It attacks a man, but Eiji Hino transforms into Kamen Rider OOO to fight it off. While pursuing the monster, he finds a near-unconscious man and takes him to the Cous Coussier restaurant, where the man introduces himself as Nobunaga, but fails to remember anything else. After visiting a library, Nobunaga starts learning at an astonishing rate. Amazed, Hino takes Nobunaga under his wing and helps him get a job at a software company.

The next day, Nobunaga meets ballerina Yoshino Akechi and falls in love with her. When the male Pteranodon Yummy attacks them, Nobunaga receives the Birth Driver from Kougami Foundation head, Kousei Kougami, and transforms into Kamen Rider Birth to fight the monster off and gain popularity. However, Nobunaga transforms into the monster Hino fought previously and causes Akechi to sprain her leg while attempting to kill her.

As a heartbroken Hino fights Nobunaga, a Greeed named Giru uses Akechi to create a female Pteranodon Yummy, who attacks Hino and Nobunaga. The former destroys it while the latter suffers from Cell Medal breakdown. Visiting a hospitalized Akechi, Nobunaga uses the last of his power to heal her before scientist Kiyoto Maki gives him three black Core Medals, which enhances Nobunaga's monstrous form. Upon confronting Nobunaga once more, Hino kills him to end his suffering. However, the black Core Medals fly off, with Hino in pursuit.

Having obtained the most intense memories of Kamen Rider battles, the male Pteranodon Yummy is caught off guard when the black Core Medals appear and combine with the Memory Memory and form the giant Kamen Rider Core. It attacks Akiko, but she is saved by Hino, who assures her Sokichi had his reasons for being a Kamen Rider before joining forces with W to fight Core. Taking the fight underground, W and Hino find a crystal powering Core near his namesake and destroy it along with Core, shattering the components that created it and causing Hino to end up in Rio de Janeiro. Meanwhile, Terui persuades Akiko to give him his equipment back so he can protect her from the male Pteranodon Yummy. Gaining help from Shintaro Goto, Terui destroys the Yummy. With her faith in Kamen Riders restored, everyone hurries back to Akiko's wedding, where Melissa arrives to take Sokichi's place and reveals he was also implanted with a Spider Bomb, which did not die with the Dopant.