Mew is one of the main characters from the movie, Mewtwo Strikes Back.
Mew first appeared at the start of the first Pokémon movie, Mewtwo Strikes Back. It didn't take a major role until the main climax of the movie. Its counterpart, Mewtwo, has been cloned from Mew's DNA by the Team Rocket scientists using one of Mew's fallen, fossilized eyelashes. Sooner or later, Mew began following everyone to New Island while Mewtwo was putting his cloned starter Pokémon to the test by battling with the trainers who attended the island. Mew followed Jessie, James, and Meowth to the main part of the island. When Mewtwo's clones were created for the ultimate battle, Mew interfered and tried to reason with Mewtwo. However, Mewtwo refused to change its mind about humans and Pokémon and decides to battle with it. During the battle, Mew kept disappearing and reappearing in the course of the battle. The battle got so intense that all of the trainer's Pokémon and Mewtwo's cloned Pokémon came to the brink of exhaustion. Just as Mewtwo and Mew were about to deliver the final blow against each other, Ash ran in between the psychic attacks and gets himself petrified by its power. All of the Pokémon and cloned Pokémon were touched by Ash's brave and noble sacrifice and their tears of sadness and grief brought him back to life. This experience made Mewtwo realize that he was wrong and decided that he will no longer conquer the world with his cloned Pokémon. Mewtwo and Mew decide to leave New Island along with the cloned Pokémon and before they disappear, Mewtwo wiped out all of the trainer's memories of this incident and transported them back to the Pokémon Center.