As Ash and his friends make their way to Shalour City, Serena and Bonnie rush off to a cave, but when Clemont asks what is so special about the cave, Serena says that they will have to wait until they get inside to find out. Once everyone is inside the cave, they all admire the particularly reflective cave walls, and Serena tells them that the cave is called Reflection Cave. Ash rushes ahead to discover more of the cave to explore. Not wanting to get left behind, the others run after Ash. While Ash wanders around the cave, he sees a bright light coming from behind him from one of the rocks, but just as he approaches the rock, the light disappears and he stares at it in confusion. Ash then approaches another mirror-like rock, which starts to shine like the one from earlier, but this time Ash sees his reflection reach out and grab Pikachu, causing him to shout out, alerting his friends as they rush to his location, but they only see him as he disappears through the opening in the mirror before it closes.
Following a bright flash of light, Ash exits the cave to find himself in a forest that looks much different than ones he has seen before. Running down the path looking for Pikachu, Ash is stopped by Serena, who is different than the Serena he knows. Bonnie also appears, informing Clemont, who races past them, then stops before running back. As Ash explains what has happened to Pikachu, Serena does not believe him when he tells them that another him grabbed Pikachu, and she and Bonnie mock him. The three are not convinced by Ash's story, but Serena does admit there is something different about him compared to the Ash she knows, and Mirror Clemont tells them of a legend about Reflection Cave. As Ash tells the three of them that they are different to his friends back in the world he came from, Serena gets annoyed and challenges him to a battle, telling Ash she will believe him if he manages to beat her, to which he accepts. Mirror Serena sends out Fennekin as Ash chooses his Froakie. Fennekin starts off with Flamethrower, which is countered by Froakie's Water Pulse. Serena then tells Fennekin to use Scratch, but Froakie uses Double Team to avoid it, making Mirror Serena panic, as the battle finishes with Fennekin being knocked out by Bubble. Since Ash won the battle, Serena has no choice but to believe he is telling the truth. Mirror Ash then reveals himself with Ash's Pikachu. Mirror Clemont asks what happened for Mirror Ash to get in this situation, to which he explains that after losing to Serena, he ran into Reflection Cave, but shortly lost sight of Pikachu, so when he saw Ash's Pikachu, he grabbed him thinking he was his. Ash then asks what Mirror Ash's Pikachu is like, to which he explains he is wild and aggressive as the scene cuts to Team Rocket trying to capture Mirror Ash's Pikachu. Pikachu uses Iron Tail to send Meowth flying into James as Jessie catches the device and captures Pikachu in it.
Ash tells Mirror Ash that he cannot leave him knowing he is in trouble and offers to help him find his Pikachu while Mirror Serena wonders how they are going to find Pikachu, to which Mirror Clemont laughs to himself. Back with Clemont, Bonnie, and Serena of the main world, they wonder how they are going to get Ash back, with Clemont revealing his new invention called the "Other Dimension Converter", though it does not work and ends up blowing up. Back in the Mirror World, Mirror Clemont reveals he has a magic spell prepared for the situation and draws a lightning bolt shape on the ground as he proceeds to put his staff in the middle and says that whichever direction it falls in will be the direction they need to search for Pikachu in. As Mirror Bonnie compliments her brother, the real world's Bonnie shouts at Clemont for his failed invention as a clearly upset Serena wonders if Ash has disappeared forever.
Back with Team Rocket, Mirror Ash's Pikachu continuously uses Thunderbolt on his cage as he tries to break free, just as James notices there are people approaching the area. Ash reveals that they have reached a dead end, to which Mirror Serena wonders where Pikachu could be as Team Rocket observe them and Pikachu as they plan to capture him as well. Mirror Ash then starts to get upset at the thought of never seeing Pikachu again, but Ash offers him some comfort. Just then, Ash's Pikachu is captured as everyone looks in shock. Ash recognizes the group as Team Rocket, but the others do not know who they are, claiming that they are known as "The Heroes of Justice", which confuses Ash, as Mirror Ash notices that Team Rocket also have his Pikachu. Ash orders Team Rocket to let the Pikachu go, but instead, they send out Inkay and Pumpkaboo. Ash then tells Mirror Ash that they should battle too, to which both send out their Hawlucha. The two Ash order their Hawlucha to use Karate Chop on Pumpkaboo and Inkay, but as Ash's Hawlucha charges in forcefully and hits Inkay, Mirror Ash's Hawlucha jogs slowly and worriedly, missing Pumpkaboo as it is tackled by it. James orders Inkay to fire Psybeam at Hawlucha, but Ash's Hawlucha steps in and takes the hit, only to become confused in the process and forcing Ash to recall it. Pumpkaboo hits Mirror Ash's Hawlucha into Mirror Ash before being returned too. Mirror Ash then starts to cry, but Ash tells him nothing will be solved by him crying and that they have not lost yet. As Mirror Clemont picks up the two cages containing the Pikachu, he is spotted by Meowth. Panicking, Clemont stands on Meowth and dodges Jessie and James's attempts to stop him as he throws the cages into the air, at which Serena has her Fennekin use Hidden Power on them, freeing the two Pikachu before they land in front of their Trainers. Ash and Mirror Ash promptly order their Pikachu to blast Team Rocket off with Thunderbolt.
After Ash is praised by the Mirror World group, Mirror Serena tells them it will be dark soon, which makes Mirror Clemont remember about the legend of Reflection Cave and that anyone stuck in the Mirror World after sunset will be stuck there forever. As Ash and the Mirror World group return to Reflection Cave, Mirror Ash explains how he opened up the portal to the other world. Ash then starts wishing hard to travel with his friends again, with Clemont, Bonnie, and Serena doing the same until a portal opens up on both sides, allowing the groups to see one another between the two worlds. As they wish for Ash's return, the portals move closer. Ash, with Pikachu in hand, steps in, only for the portals to begin moving away from each other again. As Ash starts to fall, Serena grabs his hand and his friends all pull him up. Everyone says goodbye to their doppelgangers as they promise to meet again, and once the two portals close, Ash states that the group from the Mirror World are going to continue their journey together and that they should do the same.
As they exit Reflection Cave, it is starting to get dark in this world too. Serena asks Ash if they will always be together on their journey, too, and Ash tells her that he would not want it to be any other way, to which Bonnie and Clemont chime in that they will always be a team. The group makes a stopover at a local Pokémon Center on the road to Shalour City. Meanwhile, Team Rocket find themselves marooned in the Mirror World following their earlier blast off. They look up at a rock and see Mirror Team Rocket, who are the real Heroes of Justice, leaving the real Team Rocket shocked at what they had just seen.