After arriving in the Unova region, Ash and Goh see a digital billboard that reveals a new ruin site, the Colossus Ruins, has just been discovered at Desert Resort. The boys immediately set off for the ruins in the hopes of finding colossal living inside. As they arrive at the large Desert Resort, Ash admits he doesn't know the way to the ruins and is even more surprised that Goh hasn't got a plan in mind. After Pikachu and Scorbunny step on the hot sand, the boys realize the desert is too hot to cross on foot.
Behind them, a Pokémon Ranger named Kira admits they shouldn't underestimate Desert Resort and asks if they have any Flying Pokémon. Ash responds by calling out his rather affectionate Dragonite, while Goh's Beedrill goes to attack with its stinger. Kira sends out Braviary, and with a rope and board, it easily takes Kira and her Meditite across the hot deserts sands. Ash and Goh follow suit, wearing Go-Goggles to protect their eyes from the sand, though things quickly get competitive after Beedrill deliberately cuts in front. Dragonite responds by speeding up and flying Ash and Pikachu high into the sky. While Ash and Pikachu try to regain their breaths, Goh announces they have arrived at the Colossus Ruins. Pikachu takes notice of the odd statues outside, and one quickly reveal itself to be Darmanitan. As it prepares to attack with Overheat, Goh throws a Poké Ball and makes a successful catch. The Rotom Phone immediately updates its Pokédex information with Darmanitan's data.
Suddenly, a voice takes Ash and Goh by surprise. Ash assumes it came from the Litwick standing in front on them, but Goh points out that its Trainer, the leader actually spoke. The leader reveals that he is leading the excavation team into the ruins of an ancient city. He shows them a stone tablet, explaining that the ancient city created a giant colossus to protect their civilization. A swirl symbol is of interest to the excavators, and Goh admits it looks familiar somehow. Another excavator announces they've just discovered another chamber. Scorbunny and Pikachu eagerly rush down the stone steps to investigate further. However Scorbunny accidentally steps on a tail belonging to a Sandile. Goh is particularly excited and checks Sandile out on his Pokédex. The Desert Croc Pokémon starts snapping its jaws, so Pikachu uses an Electric attack which does nothing to dissuade the Ground Pokémon. Goh calls for a more effective Double Kick, knocking Sandile down. He then proceeds to catch Sandile, which Ash congratulates him on.
Another Pokémon, this time a Woobat swoops the boys. Ash decides he'll catch it, but he steps on hidden panel and sets off a sand dump trap in his act of carelessness. Scorbunny is equally reckless and triggers a stone boulder, forcing them to run. They leap onto the walls for safety, only for Goh to press down on another trap panel which sets off a series of stone pillars. The boys take a moment to catch their breaths, but a passing Yamask catches Ash's attention. Ash pursues after it, only to step on another hidden panel leading the floor underfoot to open up. As Goh prepares to help his friend, Kira arrives on the scene and has Meditite use Psychic to pull Ash to safety. Afterwards, Kira officially introduces herself, revealing she travels around for mysteries and adventures. She explains that she is also interested in the identity of the "Colossus". Kira admits that she was prepared for this expedition unlike the boys, and points out that she was able to avoid all the traps which are clearly marked with symbols. The remarks leave Ash and Goh feeling embarrassed.
Kira suddenly hushes her companions, aware that something is approaching. Two Cofagrigus then appear, and Goh confidently declares this will be his third Unova catch. Kelira stops him, warning that something was inside the Cofagrigus. Scorbunny, however, leaps into action with an ineffective Double Kick despite the advice. Ash has Pikachu attack with a Thunderbolt, which frees Shane Seeker and his Shiny Psyduck from the Cofagrigus. Shane Seeker is grateful, and reveals that he wants to fill his dex with only Shiny Pokémon. While Shane Seeker shows off his progress, Goh remarks that he seems to had little success. Ash then asks how Kelira knew someone was trapped in the Cofagrigus, and she notes that the Cofagrigus were walking in a stooped way because of the increased weight.
While everyone tries to examine the large chamber for clues, Psyduck trips onto one of the panels. Kira is surprised to see the central floor spin seemingly into place like a puzzle, and calls for everyone to find more of the emblem panels. They soon find another four throughout the room. Kira deduces that with five switches in all, each must correspond to the five circles on the floor pattern. She orders everyone to press the other panels, and this causes the floor display to rearrange into the same familiar emblem as one on the panels and ancient tablet. A large egg-like structure emerges from the floor and breaks open to reveal a giant Golurk. Goh announces that Golurk are normally 2.8 meters tall, while this one is over five meters. The leader suddenly rushes into the chamber having finally solved the stone tablet, and his eyes twinkle at the sight of the giant Golurk.
Golurk suddenly attacks with a flash of light, blasting everything in sight. The leader notes that the tablet detailed that Golurk was a defense mechanism and was only capable of attacking. While everyone shelters from Golurk's power, Goh remarks that it is still a Pokémon. He suggests they fight Golurk in a Raid Battle. Ash is immediately in, while Kira, the leader also agree to help. Shane Seeker tries to scamper away, though Kira holds him back, even promising to introduce him to one of her friends who has a Shiny Pokémon. Hearing this, Shane Seeker and Psyduck join in. Ash makes the first move, but Pikachu's Thunderbolt is ineffective against the part Ground-type. Goh is unimpressed by the amateur tactic and reminds Ash to settle down. He is equally unimpressed by his impulsive Scorbunny, which tries a Double Kick, a Fighting move that is useless against a Ghost Pokémon. Scorbunny is infuriated by this and kicks Golurk in a fury, only to be flicked into a wall by Golurk. Golurk is also able to withstand a Fire Spin, Psyshock and Hydro Pump combo effort from Shane Seeker and the others. Golurk fires off another attack which forces Kira and Shane Seeker to recall their Pokémon. Ash has Pikachu strike Golurk on the head with an Iron Tail. While the colossus kneels down, it delivers a powerful punch that knock Pikachu out as well.
Next, Ash calls on his Dragonite. It uses a Dragon Claw to block an oncoming fist attack. Goh follows suit, sending out his Beedrill for a flurry of Twineedles. However, despite being small and agile, it is soon struck down. Kira and the leader have their Braviary and Lampent attack with Air Slash and Flame Burst, respectively, but they are quickly defeated by a Hammer Arm each. Goh makes his next selection, his newly caught Darmanitan. Golurk suddenly transforms by withdrawing its hands and legs into its body. The leader fears this is the ultimate attack described in the ancient tablet. Ash and Goh are unfazed, and opt for an equally devastating Hurricane and Overheat combination. The attacks collide, with the force blasting everyone aback. As the dust clears, Kira notices that Golurk is seemingly paralyzed from the blow and collapses to the ground. Goh takes the chance and tosses a Ball towards it. The Ball stops moving soon after, meaning Goh has now caught the giant Golurk. Everyone congratulates Goh while Dragonite and Darmanitan exchange a fist bump. The stone walls unexpectedly fall away to reveal a golden corridor behind. The tour guide is thrilled by the important archaeological discovery, while Shane Seeker hopes he will discover some golden Pokémon and Kira looks forward to the adventure ahead.
Later, Goh and Ash have since returned to the Cerise Laboratory. Inside the Cerise Park, Ash rushes inside, only to told to keep quiet by Goh. Goh points up to a hill, noting Darmanitan, in its Zen Mode, and Golurk are quickly making themselves at home as the guardians of the Park. Meanwhile, a familiar Gengar is watching from a distance.