"A Pup in Sheep's Clothing" is the second segment of the 11th episode in Season 3 of PAW Patrol.
When little lambs keep running away from Farmer Al's pasture, it's up to the PAW Patrol to find the missing sheep and discover who's been making holes in the fence. Super Spy Chase uses his spy gear and tools to search for the culprit from outside the pasture, while Marshall infiltrates the flock by dressing up like a sheep. Despite the pups' efforts, three lambs manage to escape and get trapped in a cave. Now Marshall a.k.a Baaaarney, must save them and bring the real lambs home.
Ryder, Rocky, and Marshall are on their way to Farmer Al's farm to help him out as it seems that the fence around the pen, where the lambs are kept in, has been broken and needs to be repaired. While Ryder and the pups help fix the broken fence, Farmer Al deals with shaving the adult sheep of their wool and awarding them a gold star sticker for behaving as he shaved them. This gets Marshall's attention, and Al provides sticker books for Ryder and the pups to take back to the Lookout.
Back at the Lookout, Ryder has decked out Marshall in a ton of stickers, but while Marshall scratches some of them off, Ryder gets a call from Al, revealing the fence has been busted again, and another lamb has wandered off. Ryder assures Al he'll find out who's busting the fence and summons the pups. Marshall does not watch where he's going and slips on the sticker books, leading to him crashing into the other pups and covering them with stickers. Despite that, they all share a good laugh before heading topside to get their orders. Ryder selects Chase to help him locate the missing lamb with his UAV drone, Rocky to mend the fence again, and Marshall to go undercover to find out who's making the holes in the fence for the lambs to get out. The team deploys, and once at Al's farm, Ryder and Chase set off to find the lamb while Rocky helps get Marshall ready for his undercover job, using four empty soup cans that Marshall paints black and a pair of socks to help disguise Marshall as a sheep, while also using the spare wool from the adult sheep that Al shaved earlier to further his cover.
Using the name of "Barney" with the first part imitating a sheep's bleat to get in, Marshall starts befriending the lambs, but while he is doing so, Garbie watches from outside the fence with seething anger, with her focus being on the old tire the lambs and Marshall are playing with.
Back with Chase and Ryder, they soon find the lamb and return it to the pen, where Rocky has repaired the fence once more. Marshall, meanwhile, has nothing to report on who's breaking the fence yet. However, as Ryder and Chase patrol, Marshall contacts them to reveal that the fence has been breached again, and three lambs have escaped and wandered off towards the nearby cliffs. As Chase and Ryder head off to pursue them, Marshall follows as well while Rocky gets to work fixing the fence a third time. As Marshall sets off, he trips, and Garbie bites off part of the wool in his disguise. Seeing that his cover's blown, Marshall explains himself to Garbie and tells her to back off since he's on important PAW Patrol duties, but after he leaves, Garbie still looks annoyed. Marshall hitches a ride on Ryder's ATV as they go after the lambs.
By the time they catch up, the lambs have wandered into a cave too small for any of them to follow, but Marshall, due to having befriended the lambs, is able to coax them out, but the ledge crumbles underneath them. Luckily, Chase catches all of them in his net and pulls them to safety.
Back at Al's, as Marshall, Chase, and Ryder return with the lambs, Rocky says he's fixed the fence for good this time, but a loud noise of wood splintering draws everyone's attention. It turns out, Garbie is responsible for the fence being busted so much as she only wanted the old tire in the lambs' pen as she lies down on it to take a nap. Seeing that Garbie was responsible and it was only for the old tire, Al decides to move it out of the lambs' pen once Garbie wakes up so Garbie won't bust the fence anymore. With the mystery solved, Al gets back to work with shaving the adult sheep, but accidentally shaves off part of Marshall's disguise. Marshall reminds him that he's a pup, not a sheep, leaving everyone with a good laugh to end the episode.