It's Halloween time in Adventure Bay, and Mayor Goodway is getting ready for the big party at City Hall with Marshall's help, during when his cannons were still active, he creates a large, sticky, orange spider web, which Goodway likes. However, when Mayor Humdinger and his kittens show up, Goodway is quick to tell him off with how the treats are for the kids only. Because of this, Humdinger plans to ruin Halloween for Adventure Bay.
That night, as the kids are trick-or-treating with Mr. Porter and his grandson, Humdinger and his kittens are disguised as a large spider robot, and are quick to suck up all the candy provided by the adults of the houses they visit. Fearful there won't be any candy left for them, Alex pleads to his grandfather to call the PAW Patrol.
Getting the call for help from the children and Mr. Porter, Ryder summons the pups to the Lookout. After briefing them and requesting Chase and Marshall's help, the team set off to capture the spider-bot.
They catch up just as Humdinger has hit City Hall for all its candy. Once Marshall exposes Humdinger using his EMT gear's x-ray screen, Humdinger attempts to escape, but is caught in the web that Marshall made earlier.
Soon, the Eagle appears overhead, and realizing that it might believe Humdinger's spider to be the real deal, the kittens flee, while Ryder requests Skye and Rubble for additional support. When they arrive, Skye scares off the Eagle by ringing City Hall's bell, while Rubble prepares to help Humdinger's robot spider off the web. Before he can, Humdinger spots one last piece of candy and sucks it up, but this proves to be one piece too many for his spider-bot to hold, and the bag containing all the stolen candy bursts, scattering it everywhere, much to Humdinger's disbelief and frustration. Goodway tells Humdinger that trick-or-treating is for kids only and not adults.
As punishment for his actions, Humdinger is forced to hand out candy to the kids when they arrive at City Hall for the party, with Goodway making sure he does it, while everyone shares a shout of "Happy Halloween".