Mewtwo Strikes Back & Mewtwo's Origin
This page will discuss the differences that occurred when the first ever Pokémon movie was translated, dubbed, or adapted into different versions. I will primarily focus on what 4Kids did with the English dub of the movie, since it is the version I am most familiar with, and, besides, other languages mostly used the English dubbed version as the source of their own localized versions, rather than going back to the original Japanese (alas).
I will differentiate between the Japanese characters and the dub counterparts by using the Japanese and the English names, respectively.
Some major differences are listed here, but 4Kids altered just about every line in the movie in some way. I suggest you also read through the side-by-side comparison of the English and Japanese scripts to see all of the changes that were made.