It's a clear day at Adventure Beach. While Chase is on his lifeguard duty, Marshall, Rubble, Zuma and Skye all play a fun game of volleyball. When the ball is passed to Marshall, he hits it a little too hard, sending it barely past a seagull, pass two beach-goers and into a sand castle, but is stopped by Ryder before the ball hit it. Marshall then sighs, saying that he just can't hit the ball straight. Ryder encourages him, mentioning that he just needs to keep his eye on the ball and concentrate and that he would perfect it soon. Ryder then passes the ball to Rubble and Zuma and they all continue their game of volleyball. As they played, Mayor Goodway and Farmer Yumi walk down to the seaside in their scuba gear. Mayor Goodway asks Chase about the condition of the water. He then replies that it is in perfect condition for scuba diving and to remember to always stick to each other. Back at the game, Zuma sends the ball to Marshall but when he hits it, it sends the ball to Chase, almost knocking his lifeguard chair over. Marshall rushes to Chase to check whether he is okay, and also tells himself to remember what Ryder suggested, with Chase adding, "and remember not to hit your pup-pals."
Meanwhile, Mayor Goodway and Farmer Yumi finalise checks on their scuba gear in the water and off they went to explore the depths of the sea. As they explored the aquatic life, they bump into a sunken sloop. Mayor Goodway explains that a sloop is a kind of ship pirates used to sail in. As Farmer Yumi swims closer for a better look, she is suggested not to do so by Mayor Goodway as she mentions that sloops had mysterious creatures living in them according to legends foretold. A mysterious creature's eyes then looks out of the sloop's windows, surveying its surroundings. Nearby, Sid Swashbuckle the Pirate and Arrby are seen in their pirate ship looking at the sunken sloop. Sid exclaims that he needs the sunken sloop in his personal collection with Arrby agreeing but then asks why he needed it. Sid explains that the sunken sloop must have some amazing treasure and fine pirate hats. Arrby is bewildered and questions on retrieving soaking wet pirate hats, but Sid, surprisingly, comments that he still wanted a wet pirate hat. He then asks Arrby to suit up and retrieve the sloop.
As Mayor Goodway and Farmer Yumi watch the aquatic animals around them swim, they spot Arrby. Mayor Goodway asks Farmer Yumi what it was and wonders whether it was a dog fish. She answers that it was just a dog but in a diving suit and also questions why it was tying a rope to the sunken sloop. Soon, they are left shocked as Sid (Unknown to them) begins dragging the sunken sloop along with him with Arrby following closely behind. As they dragged the sunken sloop, the mysterious creature becomes agitated. Upon realising that Sid and Arrby were taking the sunken sloop with them, Mayor Goodway and Farmer Yumi both decide to contact Ryder and the PAW Patrol.
Ryder receives the call and Mayor Goodway reports about the sunken sloop being stolen. Ryder identifies the ship captured by Mayor Goodway and concludes that it was in fact Sid's pirate sub. Ryder then calls the pups to the Sea Patrol HQ with the usual Marshall shenanigans where Marshall accidentally trips on the ball they were playing with and sends it to the other pups, knocking them over like bowling ball pins. He also states that he should maybe take up pup-bowling instead, making everyone laugh. The pups suit up and Ryder begins briefing them on their mission. Ryder deploys Rocky to use his underwater torch to cut the cable connecting the sunken sloop to Sid's submarine to release it and Chase to use his vehicle's winch to tow back the sunken sloop to its original location. Ryder then asks Robo-Dog to load the needed vehicles into the Sea Patroller and they soon begin sailing to their destination.
When they reach the sloop's location, he deploys Chase out and mentions that him and Marshall would meet him in the Sub Patroller. Ryder and Marshall then enter the Sub Patroller and began making their way to Sid.
Meanwhile, Sid's submarine begins to slow down. Sid questions on it and both Sid and Arrby rush to the window to view what was going on. Mayor Goodway then contacts Ryder to say that both Farmer Yumi and herself have been following the sunken sloop and has noticed that the sloop was actually pulling back on Sid's pirate submarine. Ryder comments that it looked as if it was a tug-of-war. Marshall then asks how the sunken sloop was able to do so and Ryder immediately notices that there was in fact a giant sea slug in the sunken sloop, finding the root cause of the pull back and commenting that it may have been using it as a shell and as a home. Sid then realises the sea slug and is angered by it hindering his great plan. However, Arrby suggests that Sid could instead let the sunken sloop go for the sea slug to keep but Sid comments that he would not let go of his pirate treasure and that there could be more treasure awaiting them. Sid then orders full speed ahead.
Back at the Sub Patroller, Ryder calls Sid on his pup-pad and asks him to give back the sloop but he denies. He then contacts Rocky to ready his torch to cut the wire and Chase to use his pail of Squid Jerky to keep the sea slug calm. He contacts Sid again and tells him that he was going to cut the cable and that he would need to stay still or else he would be ricochet back, making it dangerous for him and Arrby. Sid agrees and Ryder ends the call. However, behind his back, Sid disagrees and decides not to stop, mentioning that once the sea slug stops pulling, he would pull the sloop at full speed, pushing the sea slug out and getting the sloop.
Meanwhile, Chase begins to lower the pail of squid jerky using his vehicle's winch down to the sea slug. As the sea slug continues to pull against Sid's submarine, it notices the pail and stops to chomp eat whole. Ryder then asks Rocky to proceed with cutting the cable while it was eating and he does as told. However, as he begins cutting the wire, Sid notices that the sea slug had stopped moving and orders Arrby to speed up the submarine, but Rocky had just finished cutting the cable, thus causing Sid and Arrby to be ricochet back in a spinning motion. Ryder then asks Chase to lower his winch and Rocky to connect the winch to the sail of the sloop. With the winch secured, Chase begins pulling the sloop back to its original location with Mayor Goodway and Farmer Yumi offering to show Chase the way.
As they proceed to return the sunken sloop to its rightful place, Sid and Arrby continue spinning from the impact of the cut cable and hits an undersea rock. Determined, Sid returns to the ship's controls and tries to stabilize it. However, when he asks Arrby to activate the rudders to go forward, Arrby reports that one of the rudders was damaged possibly due to impact on the rock and that they would not be able to steer. As he finishes his report, the submarine begins to spin uncontrollably.
As they spun, Ryder, the pups, Mayor Goodway and Farmer Yumi successfully finds the sloop's original location and returns the sloop back with a jubilant sea slug. As Ryder drives the Sub Patroller back to the Sea Patroller, Marshall spots something spinning on the submarine's sonar. Ryder notices it and decides to check it out as it may be something in danger. Ryder then pilots the Sea Patroller as quickly as possible with Marshall commenting that he was piloting the submarine with perfect precision and that he must have perfect aim. They soon reach the sonar reading's location and to their disbelief, it was Sid's submarine spinning uncontrollably. Marshall also notices that the Sid's submarine's rudder was damaged. Ryder immediately calls Sid and asks him to turn off his engine as his rudder is damaged. With Arrby's comment and their upset stomachs, Sid finally agrees and turns the engine off. However, the submarine continues spinning uncontrollably. Ryder then deploys Marshall to launch the Sub Patroller's grabber to Sid's submarine to stop it from spinning. Marshall starts to doubt his aim but is encouraged by Ryder to focus on the submarine and concentrate. Marshall misses on his first try but is urged by Ryder to keep trying. With confidence, Marshall launches the grabber and is successful on his second try, stopping Sid's submarine from spinning. Ryder then praises Marshall.
When Ryder and Marshall go back to the sunken sloop, Mayor Goodway thanks Ryder and the Pups for saving the diving spot and the sea slug's home. As Ryder says his usual quote, Sid and Arrby quickly drive their ship to the sloop to retrieve it once more. However, the sea slug notices their ship with Sid and Arrby on it and knocks it forward, making the ship fly forward and skip into the horizon.