Adventure Bay is abuzz with activity as Mayor Goodway prepares to award the Key to the City to Ryder and the PAW Patrol. However, Sid Swashbuckle and Arrby are looking to loot everything they can on land as Sid believes he's looted everything he can at sea. Spotting the Key to the City, Sid plans to steal it so he can use it to loot and plunder Adventure Bay of everything he can get his hands on. To that end, while the PAW Patrol are at the ceremony, Sid and Arrby head to the Lookout, and when Arrby triggers the transformation of Rubble's dozer, it gives Sid a sinister idea. Calling his pirate crabs for assistance, Sid takes Ryder's ATV for his own, while the crabs take over control of Skye, Rocky, Zuma, and Marshall's vehicles with Arrby handling Rubble's. Armed with the PAW Patrol vehicles, Sid heads back to City Hall.
Mayor Goodway is heading to put the Key to the City on display when Sid arrives, and using Marshall's firetruck against her, gets the Key while her cellphone lands in a nearby trash can. Having Arrby smash the glass with Rubble's dozer, Sid gets the Key, and begins plundering Adventure Bay, having the crab piloting Skye's helicopter steal Mr. Porter's pies, Arrby moves the beach equipment right in front of Cap'n Turbot and Alex, and the treasure chest from the museum, to Francois' horror.
Meanwhile, Ryder and the pups are with Katie and Cali when the Mayor calls them with the other adults to tell them of the situation. Puzzled by how their vehicles are involved in the thefts, Ryder summons the pups back to the Lookout. When they arrive, they are stunned to find all their vehicles, save for Chase's, are missing. Once topside, Ryder declares the situation an Ultimate Rescue with Chase in command since they still have his police truck, and once Chase has his truck converted into the Ultimate Police Truck in the underground maintenance area, Ryder mans his police ATV, and the team heads out to solve this mystery.
Later, as they learn from Katie that someone broke into the Pet Parlor and kidnapped Cali, following the trail of Sid's looting spree, Ryder then uses his radio to identify Sid, and with Marshall activating his firetruck's siren, Skye is able to locate them from the air in her police helicopter. With the Ultimate Police Truck in pursuit, Sid has his team split up, only to find that Chase is still after him. After Chase is forced to break off pursuit when Sid uses Rocky's truck against them, Skye continues the chase, but is forced to break off herself when she saves Farmer Al when he was put in danger. Despite losing the trail, Ryder gets an idea, and when Mayor Goodway contacts them, knowing that Sid will eavesdrop on their conversation, mentions a gold mine in Dew Drop Canyon that Sid hasn't hit yet. Falling for the ruse, Sid sets course for the mine.
Once there, he unlocks the mine, claims the gold nuggets lying there in plain sight, only to then be lit up by the Ultimate Police Truck. As he, Arrby, and the pirate crabs flee, the PAW Patrol are able to take out each of the crabs one by one. Skye uses her police helicopter to dizzy the crab piloting her normal helicopter, Marshall dupes the crab driving his firetruck into spinning out Rocky's recycling truck, and Rocky and Chase disable Marshall's firetruck by puncturing the tires with a spike strip. Skye scoops up the pirate crabs with a basket, and with three of their vehicles reclaimed, the team go after Arrby and Sid, who decide to retreat back to the dock where their sub is parked with the remaining crab driving Zuma's hovercraft already waiting there.
However, once there, Sid discovers that the nuggets are fake, just rocks painted with gold paint, courtesy of Marshall, and before he can stop Arrby, he dumps the load into the barge, sinking their sub. With their main means of escape sunk, they commandeer Zuma's hovercraft, kicking out the pirate crab that was driving it, and by the time Ryder and the Ultimate Police Truck arrive, Sid and Arrby make their escape. Luckily, angry at being abandoned by Sid, the pirate crabs are more than happy to go get some payback on Sid and Arrby, swimming after Zuma's hovercraft, hopping aboard, and giving the two a good pinching. However, with the hovercraft out of control and heading towards The Flounder, Ryder has Zuma take Chase's police boat, stored in the back of the Ultimate Police Truck, to stop it from hitting Cap'n Turbot's ship. When Zuma does, he flings Sid and Arrby over The Flounder, causing Sid to drop the Key in the process, where it lands on Cap'n Turbot's head. Once the Cap'n discovers it and passes it to Skye to take back to City Hall, having lost everything and his plan ruined, Sid swears revenge, but he and Arrby are forced to swim for their lives as the pirate crabs attack them again.
Back at City Hall, Ryder returns the Key to Mayor Goodway, who intends to put it in the vault where it will be safe for the time being, and presents Ryder with a special gold-painted heart-shaped stone as thanks. When Marshall notices that he missed a spot on it, he attempts to fill it in with his paint cannons, but accidentally splashes Ryder in the process, leaving everyone with a good laugh afterwards.