In the Galar region's Iron Ruins, Quillon confronts a Registeel, his Urshifu pummeling it into submission with Close Combat as he prepares an Ultra Ball. Meanwhile, at the Cerise Laboratory in Kanto, Goh receives a notification of the very last Project Mew Trial Mission. Of the two given options, he chooses the one that requires catching Regieleki and Regidrago. Professor Cerise tells Goh and Ash that they're two mostly unknown Pokémon somehow linked with the Split-Decision Ruins in the Crown Tundra. To Ash's surprise, Goh decides to head for this mission alone, giving Ash more time to focus on training for the Masters Tournament.
After arriving in the Crown Tundra, Goh travels through Roaring-Sea Caves, Frigid Sea, and Three-Point Pass before reaching the Split-Decision Ruins. At the entrance, he finds Gary, who's also here for the same Trial Mission. The door of the ruins is sealed tight, with ancient text written on it. Quillon arrives at the scene and translates the text for them, which dictates that the door will be opened if the titans of rock, ice, and steel are gathered at the site. Throwing out three Ultra Balls, he reveals he has acquired said three titans: Regirock, Regice, and Registeel. The titans react with the door of the ruins, opening the passageway to the two other titans hidden inside. Before Goh and Gary head in, Quillon informs them that they'll only be allowed to use two Pokémon each during this mission.
While traversing through the ruins, Gary reveals that he watched Goh's previous Trial Mission, noting that he seems to have learned more about teamwork. He then asks Goh why he wants to meet Mew. Goh explains that he met Mew when he was young, and he wishes to know it better so he can learn more about Pokémon in general, given that Mew contains the genes of all Pokémon. Both of them note that this seems very much like Gary's goal of researching everything related to Pokémon, which Mew is an important part of.
The two finally reach the main part of the ruins: a big chamber with two statues and sets of circular panels on the floor. Goh recalls the patterns he saw at the entrance and figures they must've been a hint. Stepping on some of the floor panels, Gary sees them lighting up, but when Goh steps on the other set of panels, the panels on Gary's side stop glowing, indicating that only one set of panels can be active at a time. For a while, each of them tries to outdo the other, but this only results in neither of them making any progress at all. On Gary's suggestion, they try stepping on the panels at the same time, which turns out to be the key to lighting up both sets of panels at the same time. However, before they can step on their respective last panels, each of them finds themselves surrounded by giant icicles from a Frosmoth and a Galarian Darmanitan, their Trainers being Lyla and Shunya, another pair of Project Mew challengers, whom Goh recognizes from Sea Mauville. Fortunately, Goh and Gary had both set up a backup plan in case of unforeseen complications and escape with the help of Cinderace's Blaze Kick and Hatterene's Psycho Cut.
To avoid exhausting their limited Pokémon numbers more than necessary, both sides agree on a Tag Battle to decide who gets to confront Regieleki and Regidrago, with each Trainer using one Pokémon. Lyla and Shunya choose Sandaconda and Runerigus, while Goh and Gary choose Cinderace and Blastoise, respectively, with Gary explaining that he chose to use Blastoise to cover for Cinderace's weaknesses. Sandaconda and Runerigus start the battle by using Sand Tomb on Cinderace, but Blastoise uses Hydro Pump on the sand to free its partner. The next attack comes in the form of a double Bulldoze, which Goh and Gary avoid by having Cinderace climb onto Blastoise and having Blastoise launch itself into the air with Water Pulse. Cinderace then kicks the Water Pulse with Blaze Kick, sending the attack at Sandaconda and Runerigus and defeating them. While Shunya wants to keep going, Lyla honors the deal and admits defeat, though Gary sees through her seeming kindness and understands she just wants to save their strength in case he and Goh fail to catch Regieleki and Regidrago.
With the obstruction out of the way, Goh and Gary continue from where they left off, stepping onto the last panels. The patterns they inserted appear on the statues in front of them, which then shatter to reveal Regieleki and Regidrago. Goh sends out Flygon to confront Regieleki, while Gary uses Hatterene against Regidrago. While Goh and Gary's Pokémon have type advantage on their side, their opponents suddenly switch targets and attack Flygon and Hatterene with their signature moves, Dragon Energy and Thunder Cage, respectively, which neither Goh nor Gary has seen before. The titans seamlessly switch targets again and defeat their respective opponents with Extreme Speed and Hammer Arm, leaving Goh and Gary with just one Pokémon each.
Calling out their respective starter Pokémon, Goh and Gary attack the titans again, but they still stand. Regidrago summons a massive Twister, intending to defeat both of its opponents simultaneously. Lyla and Shunya chime in with an advice and tell Goh and Gary to use the Twister to their advantage and have Cinderace ride Blastoise like before. The advice is put to use as Blastoise climbs the Twister with Rapid Spin while Cinderace clings to its back. After building up momentum and height, Cinderace and Blastoise attack Regieleki and Regidrago, the increased momentum being enough to slam the titans into the chamber wall. Goh and Gary, not wasting a second, toss Poké Balls at the Legendary Pokémon and successfully catch them. Gary thanks Goh, Lyla, and Shunya for their help, telling them how cooperating with one's rivals can be useful if your goals align with theirs, while also noting that teaming up with someone unskilled simply out of friendship can drag you down. He reveals he learned this lesson from Quillon the hard way after almost getting disqualified during a mission.
As the four challengers exit the ruins at sunset, they're greeted by not only Quillon, but Danika as well, signifying that the other mission has ended as well. All five titans are sent out to stand side by side, Quillon expressing his surprise that both titans of these ruins were successfully caught. The titans start reacting to something, and a pillar of purple light emerges from behind the hills, followed by the appearance of a Dynamax Regigigas. After looking down on the group for a few moments, Regigigas disappears as suddenly as it had appeared. The group barely has any time to process what they just saw before they all receive a phone call from Professor Amaranth, announcing the final Trial Mission results and the challengers who will become Chasers. The leading challenger is Gary with a total of 15 tokens, three of which were earned from his last mission. The second chosen Chaser is Horace with 11 tokens. After hearing who the third and final one is, Goh calls Ash to excitedly announce that he passed. Gary, though, tells Goh that they've only reached the starting line, so they must still work hard. With this in mind, they both head towards whatever adventures Project Mew has in store for them.