Four isekai protagonists get dumped into a world that's designed to mock everything they think they know. The comedy doesn't come from explaining the in-jokes to newcomers, it lives in watching Kazuma, Subaru, Tanya, and Lich King stare at echoes of themselves and have no idea what's happening. The movie knows you've watched all four shows, and it's built entirely around that, which means if you haven't, you're locked out on purpose.

A homeroom class tumbles through a dimensional rift into an unfamiliar realm. What they find is a desolate wasteland under the control of a rampaging magical automaton, and some of the people they meet there seem oddly familiar. A girl-type golem carries herself much like Megumin. A woman wears the same military uniform as Tanya. A man with a cane appears to know Subaru. From there, the encounters turn into comic hijinks. The film brings the crossover cast of isekai heroes, drawn in a cute superdeformed style, into a fantasy comedy adventure on the big screen.