POKÉMON SEASON 1 EP 12

Ash, Misty, Brock, and Pikachu become the victims of pranks played by a gang of rogue Squirtle known as the Squirtle Squad. They end up falling down a hole, and are even eventually captured by them. It turns out that they are working together with Team Rocket. When Pikachu is severely injured, the lead Squirtle lets Ash out to get medicine from town, but must return soon or his friends will be trapped by the Squirtle Squad forever. In the end, the Squirtle Squad saves the town from a fire started by Team Rocket and the lead Squirtle joins Ash on his further adventures.