Stop or My Dog Will Shoot!

"Stop, or My Dog Will Shoot!" is the 20th episode of the eighteenth season.

Plot
Homer, excited about going to, 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. Lisa crafts an escape plan with, 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 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’s teamed with Lou.

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. The Simpsons send the dog away to live with Lou.

As a replacement, Marge buys Bart a huge pet, 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.

Casting
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 who did not appear in the final episode. For the episode "Homerazzi", and  were announced as guest stars but neither appeared.