Ash and Goh ride a Wailmer across the sea to Sea Mauville, the site of Goh's newest Project Mew Trial Mission, where they're greeted by Danika. When Ash tries to enter with Goh, Danika stops him, stating that this mission is for challengers only. Goh picks Inteleon as his Pokémon of choice for the mission and asks if Gary is inside already. Danika, however, informs him that Gary is on a different trial site, making Goh dismayed, as he was looking forward to facing him. Danika interprets this as Goh not considering any other challenger a threat and warns him not to take things that lightly.
Once Goh has arrived on the lowest deck as instructed, Danika contacts him and informs him that today's mission is survival: all the challengers must battle each other, with the last one standing receiving three tokens as a reward. The mission starts, with Goh's Rotom Phone displaying a map of the ship's lowest floor and the locations of all the challengers, him included. A Trainer's Granbull tries to attack him, but Inteleon defeats it with a single Mud Shot. Another Trainer approaches him, but he quickly recognizes said Trainer as his friend, Horace, who's accompanied by his Gardevoir. Just as the floor map temporarily disappears from their phone screens, Trainers with a Barbaracle and a Zangoose appear, but Inteleon and Gardevoir make short work of them, forcing the defeated Trainers to return to the entrance. Goh and Horace decide to team up, with Ash and Danika witnessing this through a holographic screen that shows drone footage from within the facility. Elsewhere, a challenger named Shunya easily defeats some other challengers before he's approached by a girl named Lyla, who offers him an alliance.
Meanwhile, Goh and Horace are wandering through the dark as they consider the possibility of hiding instead of fighting, but the map displaying their locations quickly devalues this idea. Moreover, the map shows that the remaining challengers are moving in pairs, meaning that they've understood that being in a team is advantageous to them. To prove this point, they're then approached by a pair of challengers using a Banette and a Shiftry. Banette starts attacking Gardevoir with its Ghost moves, while Shiftry targets Inteleon with its Grass moves. When Gardevoir falls down, Goh almost attacks on his own, until Horace reminds him of what results that had during their Raid Battle against Articuno. Understanding his point, Goh leaves the support moves like Reflect and Heal Pulse to Gardevoir while Inteleon attacks both of the opposing Pokémon, throwing off their strategy and soon defeating them. In another part of the facility, Shunya and Lyla also defeat a pair of challengers, with Shunya noting that any opponent can be easy if they let their guard down.
Outside, Danika presses a button on a remote. In response, several hatches open around Sea Mauville and release a multitude of Poké Balls, which open to release hordes of Jumpluff, Koffing, Roselia, and Magby to attack the challengers, with the Jumpluff and Roselia releasing Sleep Powder that puts several Trainers asleep. Danika explains to Ash that this is a way to test how the challengers will react to surprising situations, which can drag a whole team down if a team member can't deal with them and starts panicking.
Goh and Horace manage to avoid the Pokémon hordes by hiding in a small room until they've passed. Just as Horace expresses how he'd rather make friends with other people looking to find Mew than battling them, they hear a cry of help from further within the facility and follow it to find Shunya and Lyla, with Lyla claiming that her partner betrayed her and defeated her Liepard when her back was turned. Horace is ready to help her, but Goh is suspicious, since the Project Mew drone has not appeared to rule Liepard unable to battle. Further requests for help still convince Horace to defend Lyla, but as soon as his back is turned, Lyla's Liepard pounces at his Gardevoir with Sucker Punch, following which Shunya's Vikavolt attacks the Embrace Pokémon with Discharge, leaving it paralyzed. Shunya and Lyla mock Horace for being gullible and focus their attention on Inteleon, declaring that it's the final Pokémon they need to defeat, as every other challenger has been taken down. Inteleon starts battling the two opposing Pokémon, but is outmatched. When Inteleon gets caught in Vikavolt's String Shot, Gardevoir stands back up and manages to save Inteleon from being defeated by using Confuse Ray on Vikavolt and Liepard, giving the Secret Agent Pokémon enough time to free itself and allowing it to defeat its opponents with Snipe Shot and Liquidation.
With Shunya and Lyla defeated, Goh and Horace are the last challengers remaining. However, their celebration of this is cut short when Danika contacts them to remind them that only one challenger can win this mission. To Danika's surprise, Horace announces that he's decided to forfeit, as he doesn't want to battle against his friend. Goh tells him not to do so, as there's no telling if he could ever get another chance at realizing his dream to reach Celebi through Mew. As such, he convinces Horace to battle him with no holding back, which he eventually accepts. Since both Inteleon and Gardevoir have already taken heavy damage, both Trainers are aware that a single blow will decide the match. Goh tries to use Inteleon's invisibility to his advantage, but Gardevoir counters by using its precognitive powers to counter its attacks, finally culminating with Inteleon being taken down by a Moonblast, making Horace the winner. Goh lets Horace head out ahead of him, clenching his fist when he's not looking.
On Sea Mauville's deck, Danika congratulates Horace and rewards him with three tokens. In addition, Goh receives two tokens for finishing second, and Shunya and Lyla receive one each for finishing third and fourth, respectively. Quillon appears on a nearby screen, informing that, at his trial site, Gary emerged victorious, just as Danika had anticipated. Quillon declares that since the rainy season is ending sooner than expected, the Chaser selection process is being sped up by eliminating all challengers with less than four tokens. A scoreboard is displayed, showing how many tokens each remaining challenger has, with Gary being in the lead with 12 tokens. Horace, Shunya, Lyla, and two other challengers are also ahead of Goh, leaving him well outside the Top 3 who are going to be selected as Chasers.