The Pokémon Company describes the film's story:
Pokémon the Movie: I Choose You! is an origin story highlighting Ash and Pikachu's first meeting and their adventures as they search for the Legendary Pokémon Ho-Oh. The iconic pair encounter familiar faces along the way, new characters including Trainers Verity and Sorrel, and even a mysterious new Mythical Pokémon, Marshadow. Challenges and epic Pokémon battles abound in this unique new story about the beginning of one of the most beloved friendships in popular culture.
The film had its world premiere at the Japan Expo 2017 event in Paris on July 6. It then opened in Japan on July 15 and earned 516 million yen (about US$4.61 million) in its first two days to rank at #1 at the box office.