Homer, excited about going to Oktoberfest, is disappointed when he discovers that Marge has tricked him and the kids into going to a Harvest Festival which allows no alcohol. While attempting to leave, Homer accidentally leads the family into a cornfield maze. Lisa crafts an escape plan with Tremaux's algorithm, a real life maze-solving method, but Homer is left behind.
Santa's Little Helper is called upon by Bart to find Homer. The dog manages to sniff out Homer and drag him out of the maze. Santa's Little Helper becomes a hero and the Simpsons, persuaded by Chief Wiggum, enroll him in the Springfield Animal Police Academy, where he is teamed with Lou, managing to help fight crime in Springfield.
Santa's Little Helper catches Snake Jailbird who is taken to court, but freed on a technicality. This makes Santa's Little Helper jaded, and when he returns home, he bites Bart on the leg out of frustration. Considering the fact that the pressure from the fight against crime is what led to the dog's actions on Bart, the Simpsons send the dog away to live with Lou. As a replacement, Marge buys Bart a huge pet python, which he names Strangles. Bart takes Strangles to show and tell at school, where Strangles escapes into a school lab and unintentionally knocks over beakers of ethanol and nitric acid, mixing both substances and creating a toxic cloud.
Bart, being the only person still stranded in school trying to find Strangles, collapses from the smoke, (remarking "It smells like some chemicals cut one"). Both Santa's Little Helper and Strangles arrive to save Bart, who chooses his faithful dog to save him. Santa's Little Helper goes back to living with the Simpsons. Groundskeeper Willie adopts Strangles, and places him in the fire hose coil.