Pokémon S01 E68 The Evolution Solution

"Gotta catch 'em all!"

Ash goes to Seafoam Island to help Professor Oak to discover the mystery of how Slowpoke evolves into Slowbro. There, Professor Westwood V is also trying to solve the mystery. Jessie captures Shellder, and tries to rush a Slowpoke's evolution, forcing Psyduck to stall the evolution.