Mewtwo was being made in a lab to be the clone of Mew and the Strongest Psychic Pokémon. However they were only treating him like a test subject and didn't treat him like a living thing. So you sabotaged the lab causing it to malfunction and before you activated a bomb. You grabbed Mewtwo and saved him. So in a way...he is your son. After a bit of time...he finally woke up.

Mewtwo: "Where am I? What is my purpose...who are you and what do you want from me. I want answers please."