They find Wartortle and Squirtle inside their shells on the island, who upon closer inspection, Brock realizes that they are asleep. As they look farther up, there is a giant shell, which Ash’s Pokédex reveals it is a Blastoise. Ash and Squirtle check out Blastoise, and suddenly they fall sleep. Misty and Brock try to wake him up, but to no avail. Then, Misty tells Pikachu to use an Electric attack, which manages to wake Ash and all of the Pokémon up. Misty and Brock asked Ash what happened, who then replies that when he approached Blastoise, he heard mysterious music that was vaguely familiar that caused him to fall asleep. Misty and Brock look at each other as if they recognize the cause, and Misty says that they should go ahead and leave, though Ash insists they should stay to help Blastoise. Misty, shaking her head and smiling, says that she was afraid of that.
(Source: Bulbapedia)
