Here's the breakdown:
* Pokémon: The First Movie (also known as Pokémon: The Movie - Mewtwo Strikes Back) tells the story of the creation of Mewtwo, a genetically engineered Pokémon, and its quest for revenge against humanity. It's a major turning point in the anime series, introducing the powerful psychic Pokémon and its tragic backstory.
* Pokémon: Mewtwo Returns is a direct sequel to The First Movie. It takes place five years after the events of the first film and focuses on the return of Mewtwo and its renewed conflict with Ash, Pikachu, and their friends. This film picks up where the previous movie left off and explores the themes of revenge, redemption, and the power of friendship.
While they share characters and themes, they are separate movies with distinct storylines and events.