Urusei Yatsura: The Final Chapter

The fifth film is an animated adaptation of the final story of the manga and is also the official ending of the anime series, in which Lum and Ataru must repeat the game of tag played out in the first episode of the television series, or the Earth will be infested with mushrooms larger than buildings. Further, should Ataru lose, Lum will leave forever and everyone's memories will be changed so that they don't remember she, or her friends, were ever there... (Wikipedia)