One day, a woman named Chiyome Hesogakure with a child named Chinzō visits the Nohara family and claims that she is the real mother of Shinnosuke and takes him away…

After accepting an invitation from a mysterious trainer, Ash, Misty and Brock meet Mewtwo, an artificially created Pokémon who wants to do battle.

When the mysterious Shiriri turns his parents into children, Shin-chan must trek across Japan to help return them to adult form.