Mewtwo is the most powerful pokémon and the lab creation cloned after a DNA sample of Mew's discarded eyelash. Created by Doctor Fuji, he was an experiment that had many goals.
Mewtwo was young in his development process before his amazing telepathic powers allowed him to contact the deceased pokémon and even Fuji's daughter, Amber, or in this case, Ambertwo. Mewtwo's naivety and curiosity was explored as he read Amber's thought and imagined a world from the thoughts he read and explored said world as he was taught by her what everything meant, like a small child. Despite his ability to read minds and view their imaginations, he still could not understand Amber without talking to her. However, Ambertwo was not successful, and she began to die again. Mewtwo was enraged, but remembered what Amber told him; his tears made him alive, and that life was good, but he did not understand what she meant, and did not believe her. Mewtwo had to be tranquilised before his power eliminated himself and the entire lab, and as he grew more complete, he gained memories of the original Mew via his telepathy, and envied this lifestyle, and broke himself out of his containment desiring freedom and purpose to his life. When realizing he was only an experiment to humans, he destroyed them all in disgust. Giovanni, on his way to the lab to pick up Mewtwo, saw the destruction and was pleased by his power. Instead of selling Mewtwo, he wanted to control him for himself. He made a deal with Mewtwo that he would teach him to control his powers if they worked together, but the promise only lasted a short while, as Mewtwo felt the desire for more in his life, which angered Giovanni as that went against his plans to control him.
Originally, Takeshi Shudō wanted Mewtwo's story to be the finale to the pokémon series, where he would fight against Giovanni and Ash would try to soothe his angered heart. The scrapped plans eventually became the dvd sequel, Mewtwo Returns, which in America, finally had the cut origin story of Mewtwo that was in the beginning Japan's full first pokémon movie.
The Pokémon franchise belongs to Game Freak and the Pokémon Company. You may watch the remake to the First Pokémon Movie on Netflix