Released to commemorate Kamen Rider 555's 20th anniversary, the film takes place after the ending of the TV series. Masato Kusaka and Mari Sonoda are fighting a new iteration of Smart Brain led by Kitazaki, who begins an operation to exterminate all Orphnochs, with Takumi Inui seemingly working with them for unknown reasons since his disappearance.

During summer vacation Natsumi is busy studying for her entrance exams. One day, as a change of pace she plays baseball with her childhood friend, Yusho. He suggests that they invite someone else to join them but that makes Natsumi uncomfortable. The friend he was going to invite was her "former best friend" whom she had not spoken to in a while.