The episode begins with Ash, Pikachu and Greninja training at a stream at night. Ash orders Greninja to use Water Shuriken and then orders Pikachu to use Electro Ball after dodging the attack, which is then cut in half by Greninja's Cut attack, causing an explosion that sprays Ash with water, completely soaking him through before he commands Greninja to use Double Team. As Clemont and Bonnie watch Ash's training in admiration, Serena becomes worried about Ash catching a cold as she throws a towel to him, but Ash assures her that he's fine but suddenly sneezes loudly. At Team Rocket's Headquarters, Jessie, James and Meowth are kneeling before Giovanni as he congratulates them on their work in Kalos, and puts great emphasis on Meowth's achievements as he begins to reward him, which makes Meowth happy, but Meowth's impression changes as he becomes overloaded by the boss's rewards and struggles to breathe. As he continues it turns out to have been a dream as he wakes up shortly after Jessie and James shout to him, along with Wobbuffet and Inkay. As James mentions Meowth's dream he talking about in his sleep, Meowth begins to change the subject as he quickly mentions that they need to capture Pikachu. Not long after, a loud guitar is heard, catching the attention of Team Rocket as they look up to a hill and see someone above. As they watch on, the mystery person talks about how the Pikachu they're talking about can't be stronger than his before he begins to rock out once more and reveals himself to be Jimmy: The Pikachu Trainer and his Pikachu: Spike. He then asks where he can find this Pikachu they want.
Later that morning, Clemont, Serena and Bonnie are all awake, but Bonnie comments on how Ash isn't joining them. Clemont informs her that Ash was training hard the previous night, just as Serena asks Bonnie to go and wake Ash up. As Bonnie approaches Ash's tent, Ash emerges and Bonnie tells him that breakfast is ready. Ash rubs his eyes and speaks weakly before he attempts to walk to the table as Serena asks him if he's hungry. Ash begins to get closer to Serena before collapsing in front of her and holding on to her, making Serena nervous as she's so close to him. Ash attempts to talk to Serena as he tells her that "now's their chance..." managing to say "I...", which makes Serena wonder what he's trying to say, but Ash says, "Iron Tail...Pikachu" before collapsing, much to everyone's worry as Serena notices Ash has a fever and Clemont tells them they need to get Ash to bed.
With Ash now in his sleeping bag, Serena uses a wet wash cloth to help bring Ash's temperature down, but mentions that it becomes warm quickly. Clemont then comments that it must have been from the previous night when Ash got soaked and that he must have caught a bad cold. Pikachu becomes worried about Ash, but Clemont says that he'll go and get Ash some medicine and Bonnie agrees to go with him. Serena tells them that there's a pharmacy around and that they should take her guide book to help them get there and Clemont and Bonnie leave Serena to take care of Ash while they get medicine.
Pancham and Braixen gather water for Serena so she can continue to help bring Ash's temperature down. Not long after Serena applies a cold wash cloth to Ash's forehead, a loud guitar catches Serena's attention, along with the Pokémon as she exits the tent and notices a guy and his Pikachu. Jimmy asks Serena if the Pikachu belongs to Ash Ketchum from Pallet Town and mentions that he's heard it is really strong. Serena attempts to get Jimmy to keep the noise down so that he doesn't wake Ash up, but Jimmy then challenges Ash to a battle, Pikachu against Pikachu. Serena agrees to go get Ash, but once inside the tent, she realises that if she tells Ash that someone has challenged him, he'll jump up to accept it, even if he's not in any fit state to do so, and just then, Ash moves, startling everyone, but everyone is relieved when they find out he was just dreaming as he talks in his sleep about telling his Greninja to use Double Team before he begins snoring again and is back to sleep. She then notices Ash's clothes, to which she picks them up, staring at them.
Back outside, Jimmy and Spike are waiting patiently as Serena appears disguised as Ash, saying that 'he' is Ash Ketchum from Pallet Town, as all her Pokémon and Pikachu look at her. Serena and Jimmy prepare to begin their battle.
Jimmy comments on how his Pikachu Spike is the ultimate Pikachu, and Serena is relieved that Jimmy bought her disguise as Ash and tries to remain calm. Pikachu informs Serena that he's willing to assist her during the battle against Jimmy. Team Rocket watches from a nearby tree as they're impressed by Jimmy's determination to find Ash's Pikachu, but Jessie shows annoyance towards him because he thinks he can beat Ash's Pikachu. Sceptical that Jimmy can win against Ash's Pikachu, James mentions that if Jimmy's Pikachu turns out to be strong, they could capture Ash's Pikachu once Jimmy has weakened it.
Serena imitates Ash's catchphrase as she sends Pikachu into battle. Jimmy offers Serena the first move as the battle begins with Serena commanding Quick Attack as Jimmy orders his Pikachu to use the same attack. The two Quick Attacks collide and Jimmy orders Spike to use Thunder Punch, but Serena has Pikachu dodge, which proves successful. Serena orders Pikachu to use Thunderbolt this time, but Spike manages to escape using Dig. Serena begins to panic as she wonders where Spike will attack from next, but after help from Pikachu, she remembers what Ash would do in this situation and orders Pikachu to use Iron Tail, to which Pikachu slams the attack on the ground, bringing Spike to the surface and amazing Jimmy. Serena orders Pikachu to use another Iron Tail and it hits Spike hard, bringing him to the ground. Jimmy is enjoying the battle as Serena continues her Ash impression, while secretly amazed that it worked. Team Rocket continue to watch on as Jessie comes up with the idea of capturing both Pikachu if Jimmy's is just as strong as Ash's, as Meowth thinks it's a good idea. Meowth then focuses on how Giovanni will reward him if he gets the two Pikachu, only this time, it won't be a dream. Jessie snap Meowth out of his daydream, and Meowth tells them they should begin the plan of capturing the Pikachu.
In town, Clemont and Bonnie have bought the medicine and begin to make their way back to the forest where they can help Ash get better with it. Upon arriving back at the clearing, Clemont and Bonnie notice Serena battling, but mistake her for Ash at first, until Clemont looks closer and notices it is Serena. Bonnie shouts over to Serena and begins to ask why she's dressed up like Ash, but is cut off by Serena screaming to prevent her cover being blown. Bonnie then tries to ask again but Serena screams again, this time joined by Ash's Pikachu. Jimmy then asks if they are her friends and she tells him they are before she goes to Clemont, asking if he's got the medicine for Ash, to which he says he has and Ash will be better in no time, but then begins to wonder about her due to her behavior. Serena then explains the situation, which helps Clemont to understand to some extent. Clemont then tells her that she can leave the rest to him regarding Ash before leaving with Bonnie, only for the latter to return to tease Serena by wishing her luck and calling her "Ash", embarrassing Serena before she turns her attention to the battle once more, much to Jimmy's pleasure.
In the tent, Clemont gives Ash the medicine as Bonnie hopes he gets better soon. Back to the battle, Serena commands Pikachu to use Thunderbolt as Jimmy commands Spike to use Thunderbolt as well; both attacks collide causing an explosion. As both Serena and Jimmy command the Pikachu to use Quick Attack, they're stopped and caught by two mechanical hands before the moves can make contact, which shocks the two trainers. Just then, a truck tears through the foliage and is in clear view in the middle of the clearing. Two people emerge from the top of the truck on a stage and begin singing, before asking if they want to know who they are as they remove their disguises and they are revealed to be Team Rocket. With their identities now known, Jimmy recognizes them, surprising Serena as Jimmy tells her that they are the people who informed him about Ash's Pikachu in the first place. Team Rocket then reveal their plans for capturing Pikachu, but after noticing something isn't right, Jessie then questions if it is Serena dressed up as Ash, which gets James and Meowth talking about why she would do that. Jimmy, hearing everything, questions what's going on, which makes Serena nervous as she's been caught out. The two Pikachu attempt to escape using Thunderbolt, but it's no use and Team Rocket make their escape by driving off in the truck with them both. As Serena and Jimmy chase after the truck, it is suddenly stopped by Water Shurikens. Once the smoke has cleared, Team Rocket questions what just happened as Ash reveals it was him and his Greninja. Serena asks if Ash is alright as he was so ill earlier, but Ash replies that thanks to Clemont and Bonnie he is alright. Ash mentions to Serena that she battled in place of him, which causes Jimmy to ask what's going on, and Ash tells him he'll explain later as saving the Pikachu is the priority right now. With her act now blown, Serena mentions to Jimmy that she meant no harm with what she did as she removes Ash's hat, surprising Jimmy as he recognizes her as the girl from earlier. Serena then returns Ash's hat to him before the battle with Team Rocket begins as James sends out Inkay while Jessie sends out Gourgeist and both command them to use Psybeam and Shadow Ball. Ash prepares to counter by having Greninja use Water Shuriken and Serena decides to help out by having Braixen use Flamethrower, Pancham use Dark Pulse and Sylveon use Swift. Serena's battling skills catch the attention of Jimmy who comments on her skills. With the attacks countered, Ash has Greninja use Cut to free the Pikachu and shortly afterwards, Ash and Jimmy command Thunderbolt from their Pikachu, which sends Team Rocket blasting off. Meowth is upset that he won't get his reward from Giovanni now that their plan has failed, which confuses Jessie and James as the former views herself as more deserving of a reward from the boss.
With Team Rocket now dealt with, Jimmy turns his attention to Serena and tells her that he didn't appreciate being lied to about who she was, but admits that she worked really well with her Pokémon during the battle. After being thanked by Serena, he begins to rock out with his Pikachu before being reminded by Ash that Jimmy came here to battle with him, and offers to have a battle, however since Ash hadn't eaten anything all day, he was hungry and decides to postpone the battle until he's eaten, so Clemont decides to make something to eat and Bonnie offers to help. Serena invites Jimmy to join them for something to eat, to which he blushes and accepts the invitation. Everyone is around the table eating as Serena looks to the sky before she smiles happily.