Our heroes finally reach Vermillion City, and they discover that the Pokémon Center is filled with Pokémon hurt during battles with Lt. Surge, the Vermillion City Gym Leader.
Ash and Pikachu head to the Vermillion City Gym, where Lt. Surge is proud of his Raichu. (Lt. Surge's attitude is to evolve Pokémon as soon as possible.) The two Electric-type Pokémon battle, and Raichu, being the evolved form of Pikachu, delivers stronger attacks that send Pikachu to the hospital. Even Team Rocket, spying the scene from outside, is disappointed.
Nurse Joy offers Ash a Thunderstone, which is required for a Pikachu to evolve. As a Raichu, Pikachu would have a better chance of defeating Lt. Surge's Raichu, but if it evolves, it can never go back to being a Pikachu.
What will Pikachu decide? Are there disadvantages to evolving Pokémon as soon as possible? And why is Team Rocket rooting for Pikachu?