The film retells Ash and Pikachu's meeting to celebrate the 20th anniversary of Japan's first airing of Pokémon (1997).
Ash's hat appears to be a mix of his Kanto/Johto hat and Alola hat. The symbol on Ash's bag pays homage to his hat in the original series.
First Pokémon film to have Ho-Oh as the main Legendary Pokémon.
This film is considered a reboot of a few early episodes of Pokémon (1997), especially the very first episode.
The next Pokémon film, Pokémon the Movie: The Power of Us (2018), continues in the alternate timeline this movie introduced.