Pokémon: The Series Rewrite:Indigo League Part 1

episode 74: a rival in deed

Ash decides to do some training while waiting and runs into Gary and the two decide to train together for a little bit. But that's when Team Rocket attacks the two and Ash and Gary have to team up to take down Team Rocket which they do and when they return to the Indigo Plateu they learn their next match is against each other.