Pikachu had gotten tired after Team Rocket's failed attempt at stealing it. Krystal told Ash and his friends that she could lead them to a place where Pikachu could get better, but she made Ash promise he wouldn't tell where it was.
Apparently, the location Krystal knew about was a lake with a crystal in the center of it. Electric Pokémon from all around who were injured or tired swam in the lake and were cured of all ailments.
Team Rocket then tried to steal the crystal, when a Zapdos appeared and attempted to stop them. However, the legendary Pokémon ended up getting injured and was unable to be healed due to the crystal being stolen.
Luckily, Ash was able to retrieve the crystal, and with the help of Pikachu and the other Electric Pokémon, Zapdos fully recovered. Ash and the others said goodbye to Krystal and once again promised her that they would not reveal the location of the lake to anybody.