With Ash's sixth Gym Badge won, he and his friends prepare to head to Chocovine Town for Dawn's next Contest. However, the boats are unable to leave because the harbor is blocked by a school of Wailord. They have a picnic to wait out the blockade when a Luxio appears and begins to steal food from the group. Piplup and Pikachu attempt to stop the Luxio, but a Whismur and Nidoran help Luxio and manage to steal the food. Ash chases them into the sewer to find many Pokémon living in a chamber in the sewers, and that Luxio is their leader. Dawn and Brock try to follow him, but a short battle with Team Rocket leaves the manhole cover sealed shut. Brock's Happiny tries to pull it off, but to no avail. Dawn then sends out her Mamoswine but it, as usual, doesn't listen to her. Dawn bribes Mamoswine with Poffins to open the manhole cover. It charges at Dawn and stomps on the ground, opening the manhole cover and sending Dawn and her Poffins into the air. Mamoswine eats the Poffins as they fall; Happiny catches Dawn. Down in the sewers, Piplup leads the gang around the sewers to find Ash, although they often just miss each other.
Ash keeps following Luxio until he sees a Wailmer stranded on a walkway in the sewers, too tired to move. He sends out Buizel to hydrate Wailmer, and help it get into the water. He figures out that the Wailord are probably worried about it, and decide to help it out. They head up the sewers toward the ocean, though they run into a drop on the way there. With a rope and the help of the other Pokémon, they are able to pull Wailmer over the first drop, but then are blocked by another huge drop. Ash enlists the help of all the Pokémon in the sewer, making an ice ramp up the drop and pushing Wailmer up on a sled. The plan works until the ramp crumbles half-way, and the Wailmer is struggling air-born. A Venonat asks Luxio to hit him up, which knocks Wailmer the rest of the way up the drop.
Dawn and Brock finally catch up with Ash, but when they are in sight of the exit, an angry Swampert comes after them, protecting its territory. Ash tries to reason with it, but it attacks instead. He is about to have Pikachu attack, but Brock points out that electric attacks won't affect it, as it is part Ground-type. Ash calls out Grotle instead, attacking with Razor Leaf and sneaking by while Swampert is knocked out. Wailmer finally gets out of the sewers, but evolves right before he gets past a bridge. As a Wailord, he's much too big to fit under the bridge, but thankfully it's a drawbridge. Using Pikachu, Luxio and Pachirisu, they try to override the controls with electricity to raise the bridge, but they aren't enough alone. To make matters worse, Swampert has recovered and is coming after Wailord. Luxio lets out a cry, calling all the sewer Pokémon to come and join, despite not being Electric types, they can use Shock Wave. The bridge raises, Wailord gets out, and the wake of Wailord knocks Swampert back. With the Wailord reunited, they all get excited and finally unblock the harbor, allowing Ash and friends to continue.