Mewtwo Returns is the sequel to The First Movie: Mewtwo Strikes Back. It was aired in Japan as Myuutsuu: Ware Wa Kokoni Ari! (Mewtwo: Here I Am!) as a New Year's special extra long episode that was actually two episodes back-to-back.
Mewtwo Returns was released in English as a direct-to-video episode. In this 63 minute long movie, Mewtwo is shown to be living with his fellow clones in the remote and hard to reach area of Mount Quena, home to pure water springs and deadly typhoons. Giovanni, the sole living person whose memory of Mewtwo was not erased, discovers this hiding place and, together with Domino, a high-ranking Team Rocket member, seeks to reclaim his creation. It's up to Mewtwo and Ash (who just so happens to be in the area) to protect the clones as well as the pristine natural water, as well as to discover the importance of free will.
While I do not have access to the Japanese version (translated or otherwise) of Mewtwo Returns, the dubbed version, to me, seems relatively unaltered. The cloned Meowth speaks Japanese (he says "Nya" instead of "Meowth." Meowth's Japanese name is Nyasu, and "nya" itself means "meow"), Domino's name was not changed, and Mewtwo was absolutely awesome!
In this video, you are treated to the return of Mewtwo, and Mew makes a small cameo appearence. Also, the DVD version includes the uncut version of Mewtwo's Origin as an added bonus feature.