The kids have stopped at a lakeside camp site on their way to Fortree City. Ash trips and drops his badge case into the lake, where it is eaten up by a giant Whiscash. Ash dives in after the Pokémon but gets caught in some reeds instead.
Ash is pulled out of the water by a fisherman named Sullivan, who calls himself "Legend." He tells them that he has been trying to catch the giant Whiscash for years and its name is Nero. The kids decide to help "Legend" catch Nero.
"Legend" shows them how to fish, but all of them are unsuccessful. On the other side of the lake Team Rocket is also fishing, but not having much luck.
Eventually Ash snags Nero and manages to pull the giant Whiscash out of the water. Before he can get his badges back, Team Rocket scoops Nero up in a net and flies away in their balloon. Ash's Grovyle uses Bullet Seed to send the balloon crashing down, and a battle begins. Whiscash uses Earthquake on Team Rocket, and then Pikachu sends them blasting off with Thunder.
"Legend" tries to catch Nero using his own Pokémon, but eventually he just pulls out a Master Ball and throws it—which Nero gobbles up, and then swims away.