Why did Mugen Train become the highest-grossing film in Japan?

Start with what the film was. Mugen Train is not a side story. It adapts the manga's Mugen Train arc as a direct sequel to the first television season, directed by Haruo Sotozaki and produced by ufotable, with the television cast and staff carried over. The production record gives the format a practical reason: a film was judged the best format for the arc because of its shorter content and dramatic pacing. Anyone who had finished the series in 2019 had exactly one place to continue the story, and it was a cinema seat.

Then there is the calendar it landed in. The film opened in Japan on October 16, 2020, during the COVID-19 pandemic, when other major releases had been delayed. The documented effect was concrete: the number of available screens was higher than usual. Mugen Train opened in 403 cinemas, including all 38 IMAX cinemas in Japan. A hit with somewhere to play, in the same autumn the manga was selling at its record rate, met an audience with few competing premieres.

The run itself did the rest. It set the record for the highest opening weekend in Japanese cinema, at 4.6 billion yen, and became the fastest film to reach 10 billion yen in 10 days, 20 billion in 24 days, and 30 billion in 59 days. In 73 days it reached 32.48 billion yen and became the highest grossing film of all time in Japan, passing Spirited Away, which had held the record for 19 years. It stayed in the Japanese top 10 for 32 weeks, and after 220 days it became the first film in Japanese cinema history to gross 40 billion yen. Worldwide it took 512.7 million dollars, making it the highest grossing film of 2020 anywhere in the world, the first time a non American production had topped the annual global box office.