Our heroes stop at a Pokémon Center on the way to Cyllage City, where they meet a Trainer named Lena and her little sister, Lyn. It seems Lyn and Bonnie have a lot in common: they’re both too young to be Trainers, but they love taking care of their older siblings’ Pokémon. They also carry identical bags, which leads to a mix-up: as they’re leaving the Pokémon Center, Lyn accidentally picks up Bonnie’s bag, and Bonnie takes Lyn’s. The girls don’t realize their mistake until they discover the wrong Pokémon inside: Lyn has ended up with Dedenne, and Bonnie is surprised to see a Pichu peering up at her! Lyn immediately runs off to try to find Pichu, leaving Lena trying to find her!
As soon as they figure out what’s happened, Ash, Serena, Clemont, and Bonnie return to the Pokémon Center to ask around. Lyn hasn’t come back, but Nurse Joy remembers a girl with a bag that looks like Bonnie’s, wearing a red ribbon in her hair. Our heroes set off in search of the little girl, and a passerby directs them to the park where she usually plays, but they have no luck. Finally, Clemont decides that the situation calls for one of his inventions! He remembers how Pikachu and Dedenne communicated with electricity the first time they met, and shows off a device that will broadcast Pikachu’s electricity over a wide area in hopes that Dedenne will sense it and come find them.
At first, it seems to be working—but as usual, it explodes when Clemont tries to give it more power. The commotion draws the attention of Lena, who has also come to the park looking for Lyn. And on the other side of the park, Dedenne has picked up on the electricity and runs off in search of the source, with a frantic Lyn chasing after it! The two of them run into a wooded area—and right into an angry Beedrill. Dedenne bravely fights it off, but then the whole swarm descends...until an unexpected Thunderbolt scatters them! Pikachu, Ash, and the others have found the wayward Dedenne and Lyn just in time. The girls trade bags again, reuniting Dedenne with Bonnie and Pichu with Lyn, and everyone settles in to have lunch together before our heroes continue their journey!
While the group continue on their journey, Clemont reminds his little sister Bonnie not to give Dedenne too many snacks otherwise it will not eat a proper healthy meal. Bonnie interprets that she cannot give Dedenne any snacks, which alarms Serena who is giving Fennekin a twig to munch on. Ash admits he and Pikachu regularly enjoy food, to which Clemont clarifies his statement that snacks are okay every now and then. As they are nearing the next city, Serena suggests that they take a break at the local Pokémon Center. As Bonnie reaches to get Dedenne out of her bag, she realizes it is missing. While the group searches, Serena finds it staring at some fruit on a tree, being its usual gluttonous self.
At the Pokémon Center, Bonnie sees a beautiful woman and asks her to consider being her brother's wife. Clemont grabs Bonnie and pulls her away using his Aipom Arm device to save the embarrassment. As Clemont and Bonnie argue, the lady leaves with her younger sister, who accidentally grabs Bonnie's bag instead of her own. With their Pokémon fully healed by Nurse Joy, Ash suggests that he and Clemont have a battle. As they walk over the battlefield outside, Bonnie grabs the other bag, unsuspecting of the accidental swap. On the battlefield, Clemont's Chespin begins with Tackle. Pikachu responds with Iron Tail, which hits Chespin. Chespin tries a Vine Whip on Pikachu, who dodges. On the sidelines, two black ears pop out of Bonnie's bag, before Bonnie realizes a Pichu is now in her bag. Pichu is immidaitely fearful of the strangers and zaps Serena and Bonnie. To stop Pichu's electricity, Ash has Pikachu use Thunderbolt, and Pichu calms down. After using her Pokédex to identify the Pokémon, Serena asks Bonnie why Pichu was in her bag. Bonnie realizes that she picked up the wrong bag, and the group rushes back into the Pokemon Center, where Bonnie's bag is gone. Nurse Joy tells them the little girl who was previously at the Center with her older sister had a similar bag. Nurse Joy informs the group that the girl has a big red ribbon in her hair. Clemont comforts Bonnie, who blames herself for losing track of her bag and Dedenne, and the group takes to the streets to search.
Elsewhere in the city, the little girl named Lyn discovers Dedenne in her bag. While her older sister Lena talks to a friend, Lyn runs off to find Pichu, who is one of her older sister's most precious Pokémon. Ash and his friends run down the city streets, asking about the girl wearing the red ribbon. After one false alarm, the group tells Bonnie that the Pichu's owner will probably return for it, and that Nurse Joy will notify them if she does. At a city park, Lyn temporarily loses Dedenne, and soon finds it standing outside a doughnut truck. She realizes Dedenne must be hungry, so she gets it a doughnut. Afterwards Lyn decides to head back to the Pokémon Center so that she can ask Nurse Joy if she has seen Pichu. Following on a lead, Ash and his friends arrive at the park, however Lyn has already left.
Clemont comes up with an idea and after a while he unveils his new invention, entitled the "Radiowave Amplification Device," which will amplify Pikachu's electricity waves over a wide range so that Dedenne can pick them up. The machine initially works until Pikachu overloads it with his electricity. The resulting explosion gets the attention of Lena, who rushes to the park and immediately spots her Pichu. After the group explains, Lena says that Dedenne must be with her little sister, Lyn, who has disappeared, and the whole group decides to search together.
Lyn is already outside the city limits, following Dedenne, whom she thinks is leading her to the Pokémon Center. However Dedenne leads her into a vineyard. Lyn temporarily loses sight of Dedenne, but soon finds it holding a bunch of grapes in its mouth and being chased by a Beedrill. Beedrill kicks Dedenne, ripping a bow Lyn had put on its head, and Dedenne responds with a Thunder Shock, which seems to discourage the Beedrill. However, just as they think they're safe, it appears again, and this time with a swarm Beedrill in tow. Lyn trips and falls, and just as the Beedrill near her, they are struck by a Thunderbolt fired by Pikachu. Ash's group and Lena come running up the path, with Pikachu in the lead. Lyn is reunited with her sister while Pikachu and Dedenne get rid of the Beedrill with a Thunderbolt and Thunder Shock combination. Afterwards, Lyn clutches onto her sister’s leg and begins to cry as she apologizes for running off. At first when Bonnie sees Lyn crying, she looks confused, but she soon erupts into tears as well as she apologizes to Dedenne for what had happened, both the little girls being comforted by their older siblings. Ash's stomach growls, and they all laugh. As the whole group sits down to dinner, only Pichu and Dedenne aren't hungry, having been given too many treats from Lyn and Bonnie, who smile guiltily.