It's a scorcher of a day in Adventure Bay, and at the Lookout, the pups are doing their best to endure it with pup-sicles, until Marshall suggests getting the pool out for everyone to relax and stay cool in, except Rocky, of course.
Meanwhile, the Turbots are loading a large glass lens onto a trailer attached to the back of Cap'n Turbot's truck as it was the new lens for the Seal Island Lighthouse, and they had to be very delicate with it. Once loaded, they set off for the dock where the Flounder is moored, but unknown to either cousin, the hot sun is using the lens as a magnifying glass to turn its rays into a powerful heat ray, causing numerous problems around Adventure Bay, such as causing some logs being carried by Mr. Hudson on the train to begin to smolder, soften the gold in the legs of the Chickaletta statue, causing it to lean over, wilt Mayor Goodway's tulips, and cause the asphalt in the road to melt into sticky tar, trapping Ms. Marjorie and Maynard in the bookmobile. Mr. Hudson, Mayor Goodway, and Ms. Marjorie call Ryder to let him know, and realizing how big this situation is, Ryder summons the pups to the Lookout for an Ultimate Rescue. As the pups head up following Marshall crashing into them with the still empty pool, the PAW Patrol shield changes to Marshall's badge, indicating that this was an Ultimate Firefighter mission with Marshall in charge, as the pups are suited up in their firefighter gear, and Ryder dresses in his fire chief's uniform. Once they have their orders, Marshall heads down to his pup house for it to be converted into the Ultimate Fire Truck in the underground maintenance area, before roaring out once back topside to pick up the other pups and speed off to the rescue with Ryder following alongside on his ATV in its firefighter colors.
Making their way through Adventure Bay, they help out Mr. Hudson, Mayor Goodway, and Ms. Marjorie with their problems that were the result of what happened with the Turbots, while the cousins make their way up to the Lookout to ask Ryder for help getting the lens to Seal Island. However, their trailer gets unhooked from hitting a stone on the road, causing it to roll back down to the base of the hill, where the sun once again uses the lens to create a heat ray that begins to heat up the water tower, threatening to boil all the water inside into steam. It draws the attention of the PAW Patrol, and once they discover the source, Marshall has Skye drop a fireproof tarp from the Ultimate Fire Truck over the lens, thus preventing it from causing any more problems, though confusing the Turbots when they arrive to retrieve the lens, while Marshall and the others hit the water tower with as much water as they can to cool it down while Rocky opens the vent on top to release all the steam.
Afterwards, the pups help get the new lens to Seal Island to be installed in the lighthouse, and Cap'n Turbot and Francois apologize to Mayor Goodway for all the trouble the lens caused around Adventure Bay, leaving everyone with a good laugh.