One day, while playing a game of tag, Shinnosuke Nohara and his friends Toru Kazama, Nene Sakurada, Masao Sato, and Bo-chan stumble upon a decrepit, seemingly abandoned movie theater called the Kasukabeza. Inside, they find a Western film playing to an empty house, the projector running on its own. When Shinnosuke steps out to use the bathroom and returns, his four friends have vanished. Later that evening, none of them have come home. Concerned, the entire Nohara family—Shinnosuke, his father Hiroshi, mother Misae, and infant sister Himawari—return to the theater to search for the missing children. As they watch the film, a blinding flash of light engulfs them, and they find themselves standing in the middle of a desolate, sun-scorched American Old West.

The family soon arrives at a dusty frontier town called Justice City. There, they encounter Kazama, who has become the town’s brash and cruel sheriff, and has completely forgotten his life in Kasukabe. They also discover that Masao and Nene now live as a married couple, with Masao as a henpecked househusband and Nene as a saloon waitress. Only Bo-chan retains fragmented memories of their real lives. The Noharas learn from a kind girl named Tsubaki, who works for the town’s tyrannical governor, and from a film-obsessed Kasukabe native named Mike, that anyone drawn into this world gradually loses their memories of the outside world, eventually accepting their roles as characters in the film. The movie’s time has stopped because the story is unfinished, and the only way to escape is to reach the film’s ending.

To prevent this, the evil Governor Justice Love has hidden the film’s conclusion and rules the town with brutal force. As the Noharas begin to forget their own pasts, Shinnosuke resolves to fight back. With the help of an eccentric inventor named Dr. Okegawa, who has created special underpants that grant superhuman powers, Shinnosuke reunites his friends and restores their memories. Together, they transform into the costumed heroes known as the Kasukabe Boys. After a climactic battle against Governor Justice and his giant robot, they break the seal on the hidden ending, revealing the characters “O,” “Wa,” and “Ri”—the Japanese word for “The End.” The film concludes, and everyone is returned to the real world. However, Shinnosuke is heartbroken to discover that Tsubaki, with whom he had promised to return to Kasukabe, was actually a character from the film and has vanished along with the movie. As he mourns her loss, his loyal dog Shiro, left behind at home, comes running to greet him, pulling him back to the comfort of his true family and life.