Ash and friends are eating lunch in the woods when they notice that Treecko has wandered off. After failing to convince the Grass-type to come down from a tree, lunch is disrupted by a rampaging Loudred. Its Trainer, Guy finally catches up with his Pokémon and makes it stop.
Ash and Guy decide that both troublesome Pokémon need exercise, so they battle. While battling, both Pokémon evolve at the same time, into Exploud and Grovyle. The battle continues, but Exploud won't listen to Guy. It hits Grovyle hard and runs off.
Grovyle catches up with Exploud at a waterfall, and the two begin to battle again. Exploud still won't listen to Guy. Team Rocket interrupts, trying to capture Exploud, but Guy protects his Pokémon. Pikachu and Exploud send Team Rocket blasting off.
Guy and Exploud reconcile, and Ash and Guy decide to finish the battle.
The group have stopped for a lunch break as they near Petalburg City for Ash's upcoming Gym battle with Norman. Max points out that Treecko is secluding itself up in a tree and appears to be in an unhappy mood. Ash climbs up and tries to cheer Treecko up, but it doesn't work. Corphish then runs up the tree and knocks Ash away, also attempting to assist Treecko, but it again doesn't work. Max is concerned by Treecko's behavior, but Ash assures him that things will be fine. Suddenly the group begins hearing a something, and soon its source, a Loudred rampages through their picnic spot. It continues dashing along toward its Trainer, Guy. Guy attempts to calm Loudred down but can't seem to, until he jumps on its back. Next, Guy introduces himself to the group and reveals that he is on a journey to enter the Hoenn League, and Ash declares that he is going for his fifth Badge to also compete in the League. Guy is impressed, admitting that he has only earned two Badges so far. Guy says that not very long ago, Loudred has grown to be a troublemaker that sometimes has trouble listening to his commands. Max admits that he understands the irritation of traveling with someone for a while, which annoys his older sister May. Ash considers Treecko's current odd behavior, and suggests a battle would help ease Loudred's angst. Guy agrees to the idea. As Ash goes to select his own Pokémon, Treecko leaps down and is determined to battle Loudred.
Treecko commences the battle with a Quick Attack which appears to do no damage. Loudred counterattacks with a Stomp that traps Treecko, but it manages to break free by using Bullet Seed. After this, Loudred responds with a Supersonic that knocks Treecko back, and follows up with Earthquake. Treecko dodges the seismic waves and sends Loudred flying into a tree with a powerful Pound attack. After this, both Pokémon glare at each other, and start to evolve at the same time. As everyone seems to be awed and surprised that Treecko and Loudred simultaneously evolved, Ash's newly evolved Grovyle reveals that it has learned a new move, Leaf Blade. The battle continues and Grovyle charges in with another Leaf Blade, when Guy commands his newly evolved Exploud to dodge. But instead of dodging, Exploud sends Grovyle flying back with its newly learned Hyper Voice attack. After this, Exploud begins rampaging once again and Grovyle tries to stop it with Bullet Seed, but it does nothing. Grovyle then uses Quick Attack, but Exploud hits it away and continues charging, and Guy and the group then goes after Exploud. Meanwhile, Team Rocket stand near a cliff, starving and exhausted. Jessie, James, and Meowth all run to the foot of a cliff and spot Guy's Exploud drinking from a waterfall, and realize that it is the evolved form of Loudred, a Pokémon with which they have had many troubles in the past. Just as Jessie and James are about to walk away, Meowth stops them and declares that Exploud would be the perfect alarm clock for Giovanni. Jessie and James instantly have a change of heart and set their sights on catching Exploud.
The group and Guy start calling Exploud's name but get no response. As Guy begins to feel disheartened, Ash offers him hope that they'll find Exploud soon. Ash notices that Grovyle is becoming agitated, but it refuses to return and instead leaps into the trees. Guy looks up at Grovyle and is reminded of how tame Loudred used to be when it was a Whismur. Meanwhile, the Rocket trio put their plan into action with James firing corks into Exploud's orifices as it sleeps. With phase one complete, James then captures Exploud in a net. But not long after, Exploud breaks free and blasts Team Rocket off, releasing a loud cry in the process. Grovyle hears this cry and proceeds towards it. Grovyle makes it to the waterfall and stares Exploud down for a while, until they resume their previous battle as the sun comes out. Grovyle charges in, and Exploud uses Stomp. Grovyle dodges and uses Pound, only to provoke Exploud to strike back with Hyper Voice. After taking the hit, Grovyle lands a direct hit with Bullet Seed that knocks Exploud down. The rest of the group then arrives at the waterfall and the real battle begins. Ash commands Grovyle to use Leaf Blade while Guy orders Exploud to dodge. But instead of dodging, Exploud takes the hit head on. Grovyle goes in for another Leaf Blade and Guy again commands Exploud to dodge. Exploud once again ignores Guy's command and blocks Grovyle's attack, while Grovyle breaks free using Pound. After Guy scolds Exploud to listen to his commands, it walks away just as Team Rocket prepares for another ambush.
Just as Guy notices Team Rocket in the bushes, James fires five heat-seeking corks at Exploud, but Guy pushes it out of the way just in time. As the corks return toward Exploud, Pikachu uses Thunderbolt to destroy them. Subsequently, Team Rocket once again gets blasted off by Pikachu's Thunder and Exploud's Hyper Voice. Feeling touched that Guy saved its life, Exploud helps him up and it explains that it wants to continue training with him, making the two hug each other. After this, Grovyle and Exploud resume their battle once again.