Ash and Goomy are training in the Lumiose Badlands when an injured Spoink appears and leads Ash and the others to its oasis home. There, our heroes find a group of Spoink under the control of an unpleasant Grumpig, who’s using its psychic powers to control them and steal all their food!
When our heroes retreat to plan their strategy, they become trapped in a cage set by Team Rocket! It turns out Grumpig is in cahoots with the villains to seize the oasis and its impressive food supply from the resident Spoink. First, Goomy slips through the bars and challenges Grumpig. Then, when Ash frees himself and leaps into the battle, Goomy evolves into Sliggoo to protect him!
When Grumpig realizes Jessie, James, and Meowth were just using it for their own selfish ends, it angrily sends them blasting off, and then apologizes to the Spoink with a gift of food. All is forgiven, and our heroes are off once more to Lumiose City!
While traveling to Lumiose City so Ash can have his Gym battle with Clemont, the group has to make their way through the Lumiose Badlands. Noticing that Goomy appears to be coughing, Bonnie asks if it is all right, but Clemont assuages her fears and says that he believes Goomy is trying to learn a new move. The others are all impressed with Goomy, wondering what kind of a move it is, and Ash decides to do some training with Goomy. However, Goomy's practice battle with Pikachu is interrupted by an injured Spoink begging for help. After Clemont treats its injuries, the Spoink leads Ash and his friends to an oasis so they can find out what had happened to it.
When they arrive at the oasis, the group sees a Grumpig lying down by the side of the oasis, forcing other Spoink to bring it food. When one of the Spoink trips and causes the food to fall, Grumpig is enraged and throws a Shadow Ball at the group of Spoink. The Spoink traveling with Ash and his friends tells them that Grumpig caused its injury, too. Seeing the way Grumpig is abusing the Spoink, Ash decides that they are going to stop Grumpig. Goomy, also, remembers the way it was abused by a group of Bug-type Pokémon and is angry at Grumpig's actions. However, when Ash confronts Grumpig, the latter responds by hypnotizing all the Spoink and ordering them to attack Ash and his friends. Ash tells Pikachu to not strike back because the Spoink's actions are not their fault. The group flees into a cave, but are trapped there when iron bars fall down behind them to form a cage.
As Ash and the others try to figure out what is going on, Team Rocket appears alongside Grumpig and explain their plan. They had found the oasis and also saw a hungry Grumpig. They convinced the Grumpig to help them by dominating the Spoink so that it would get all the food it wanted and Team Rocket would get their share as well. Team Rocket tells Ash to hand over Pikachu, to which Ash refuses; however, Grumpig begins attacking the hypnotized Spoink instead. Team Rocket threatens more of the same unless Ash gives them Pikachu. Left with no other choice, Ash agrees and Pikachu is put into a cage. To everyone's surprise, Chespin comes out of its Poké Ball and willingly goes into the cage with Pikachu. Leaving Grumpig to watch their captives, Team Rocket returns to their balloon to load all of the food onto it. Unbeknownst to Grumpig, Team Rocket has no intention of sharing their spoils with it.
Meanwhile, Grumpig has fallen asleep. Since Goomy and Dedenne can slip out of the cell without a key, they escape and try to take the key from Grumpig. However, Grumpig catches Dedenne and defeats it in a battle while Chespin uses Vine Whip to try and pick the lock on its own cage. During all this, Frogadier uses its own attacks to try and break free some of the more brittle rocks so that they can break down the door to their prison. Seeing Dedenne's defeat, Goomy recalls its own terrifying past and decides to stand up against Grumpig. However, Grumpig is much stronger, and Goomy takes a lot of punishment from it despite putting up a valiant fight. Just before Grumpig hits Goomy with an Iron Tail, Ash manages to break down the bars confining him and takes the attack for Goomy, causing his Pokémon to become even more determined and evolve into a Sliggoo, learning Dragon Breath in the process and defeating Grumpig. Its control over the Spoink is relinquished, Chespin manages to free itself and Pikachu, Grumpig learns that Team Rocket was only using it, and Team Rocket is sent blasting off.
With Team Rocket gone, the Spoink reclaim their stolen food. With all of the Spoink united against it, Grumpig begins to show remorse, and the Spoink that befriended Ash and his friends offers Grumpig some of the fruit, allowing Grumpig to continue living together with the Spoink as a friend instead of their enemy.