"Pokemon: Mewtwo Returns" is a direct sequel to the first Pokémon movie, "Pokemon: The First Movie - Mewtwo Strikes Back." The story takes place after Mewtwo, a genetically engineered Pokémon created by humans, has erased the memories of those who encountered it and retreated to a remote mountain valley to live in isolation. Mewtwo, grappling with its purpose and identity, establishes a sanctuary for Pokémon who have been abandoned or mistreated by humans. The sanctuary is hidden within a stormy mountain range, protected by Mewtwo's psychic powers.

The narrative begins when Team Rocket, led by Giovanni, discovers Mewtwo's location and devises a plan to recapture it. Giovanni, who originally funded Mewtwo's creation, seeks to exploit its immense power for his own gain. He sends a team of scientists and operatives to infiltrate the sanctuary, using advanced technology to neutralize Mewtwo's abilities. Meanwhile, Ash Ketchum, Misty, and Brock, along with their Pokémon, are drawn to the area after encountering a group of abandoned Pokémon fleeing from the sanctuary. They learn of Mewtwo's plight and decide to intervene.

As the story unfolds, Mewtwo struggles with its distrust of humans, reinforced by Giovanni's betrayal and the scientists' aggressive tactics. The conflict escalates when Giovanni's forces breach the sanctuary, leading to a confrontation between Mewtwo and its creators. Ash and his friends attempt to mediate, emphasizing the potential for coexistence between humans and Pokémon. Mewtwo, initially resistant, begins to question its stance as it witnesses the compassion and determination of Ash and his companions.

The climax involves a battle between Mewtwo and Giovanni's forces, with Mewtwo ultimately overpowering them. However, instead of seeking revenge, Mewtwo chooses to spare Giovanni, demonstrating its growth and rejection of violence. The sanctuary is preserved, and Mewtwo reaffirms its commitment to protecting Pokémon. The story concludes with Mewtwo erasing the memories of the humans involved once again, ensuring the sanctuary remains a secret, while Ash and his friends continue their journey, inspired by the encounter.

The main characters include Mewtwo, whose internal conflict drives the narrative; Ash Ketchum, whose unwavering belief in the bond between humans and Pokémon serves as a moral anchor; and Giovanni, whose ambition and manipulation highlight the darker aspects of human ambition. The story explores themes of identity, coexistence, and the ethical implications of scientific experimentation, all while maintaining the core values of the Pokémon franchise.