When Homer gets lost in a cornfield maze, Santa's Little Helper comes to the rescue, sending the hound to animal police academy, and it's "Bad boys, bad boys, whatcha gonna do, Santa's Little Helper's gonna come for you," but when he bites Bart rather than play catch with him, Bart gets a new pet - a snake - which gets loose in the school and causes a toxic gas cloud to overcome Bart while he's looking for his canine friend.
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 bans 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. Santa's Little Helper manages to locate and drag Homer 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’s teamed with Lou.
Santa's Little Helper catches Snake Jailbird who is taken to court, but liberated on a technicality. This makes Santa's Little Helper jaded, and when he returns home, he bites Bart. The Simpsons send the pet 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 chemicals, creating a toxic cloud. Bart collapses inside the building.
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. Strangles becomes adopted by Groundskeeper Willie, who places him in the fire hose coil.
Rudy Giuliani was originally announced in the official press release as appearing in this episode. As a result, several images of him recording his part as well as a promotional image of him in Simpsonized form, were released. However, his appearance was not included in the final version of the episode.  According to executive producer James L. Brooks in an interview with Charlie Rose, the show was not allowed to use Giuliani's appearance because he had become a presidential candidate. His appearance is present in the syndicated version of the episode and the version that aired on Sky One in the UK where he gives the commencement address at the police animal academy graduation ceremony.
Giuliani's absence marks the second time during season eighteen that a guest was announced in the official press release but did not appear in the final episode. For the episode Homerazzi, Harry Hamlin and Peter Wolf were announced as guest stars but neither of them appeared.