Once he is discovered to be sick, the family rushes him to the hospital to undergo an emergency operation of bloat. Homer is saddened to tell Bart and Lisa that they just can not afford the $750 for the operation, but seeing how much everyone (including himself) loves the dog, he resolves to find a way to pay for it.

To save up the money, everyone must make sacrifices and give up their small luxuries. Among other things, Homer has to give up buying beer, Marge has to forgo her weekly lottery ticket, Bart has to have his hair cut for free at Springfield Barber College, Lisa has to forgo the 4th edition of Encyclopedia Generica (Copernicus to Elephantiasis), and Maggie's clothes have to last a little longer.

These sacrifices put a great strain on the family: in the next lottery, Marge's regular numbers would have won $40,000; Homer has to beg drinks of beer at Moe's by singing; Lisa has to do a report on Copernicus using a dog-eared third rate reference book; Maggie's clothes rip to shreds leaving her in a modified Crown Royal potato sack; and Bart suffers from an awful haircut. All the family members take their resentment out on the dog.

Feeling unloved, Santa's Little Helper runs away from home and goes off on an adventure, only to be captured, taken to the pound, and adopted by Mr. Burns, who trains him to become one of his vicious attack hounds. After a long brainwashing process, consisting of the Ludovico technique, and Santa's Little Helper being trained by attacking Smithers who is in a protection suit, Santa's Little Helper in turn becomes a bloodthirsty killer.

The family begin to regret all the nasty things they said about the dog, Bart decides that his mission is to get Santa's Little Helper back. When Bart goes to Burns's mansion to retrieve his dog, Santa's Little Helper (along with other vicious dogs) tries to attack him but remembers all the good times they had and snaps out of his brainwashed state. After that, the other vicious dogs try to attack Bart, but Santa's Little Helper growls at them to leave Bart alone. The dog then returns to the family who starts loving him again.

The episode ends with a disclaimer: "No dogs were harmed in the filming of this episode. A cat got sick and somebody shot a duck, but that's it."

[edit] Trivia

Unlike in Lisa's Pony, you can not hear Homer moaning when the rest of his family sits on him in the couch gag.
While we're tracking Santa's Little Helper's movements on the map, the road the car travels down when he gets picked up is called 'Michael Jackson Expressway', a reference to the season three premiere episode Stark Raving Dad.
Ned Flanders is seen wearing his "Assassin" running shoes from the episode Bart's Dog Gets an F.
The poster on the wall when Homer puts the lost pet notice up is Principal Skinner's "Have you seen my body?" poster from Bart the Murderer.

[edit] Cultural references

Lisa corrects Homer, indicating Nixon's dog was named Checkers.
The veterinarian bears a striking resemblance to Vince Edwards, who played Ben Casey on the 1960s drama of the same name.
Among the books that end up in the Simpson's fireplace are The Lottery by Shirley Jackson, Fahrenheit 451 by Ray Bradbury (which ironically enough, is about a society where books are burned), Fatherhood by Bill Cosby and a book entitled "Canine Surgery". Fatherhood was heavily referenced in the episode Saturdays of Thunder earlier this season.
The scene in which Mr. Burns and Smithers brainwash Santa's little helper is a parody of Stanley Kubrick's 1971 film A Clockwork Orange, complete with Beethoven's "Ode to Joy". The film uses footage of a kind that would aggravate a dog: a dog being hit with a rolled-up newspaper, a shoe kicking a water dish, a kitten playing with a ball of string, a tank running over a doghouse, a dog being hit on the head by a falling toilet seat, and finally "footage" of Lyndon Johnson holding his dog up in the air by the ears (he really did this). Interspersed with this footage are scenes more typical of the "Clockwork Orange" reference, including a nuclear explosion and "footage" of the Hindenburg disaster.
Music from Peter and the Wolf, a children's story composed by Sergei Prokofiev, is played over the wanderings of Santa's Little Helper through Springfield's outer domains.
This episode contains some references to facts or rumors about Michael Jackson; for example, Kent Brockman's butler telling Kent that his pet llama bit Ted Kennedy, and Mr. Burns is sleeping in an iron lung as part of his longevity treatment.
A quick cutaway shows that Mr. Burns lives on the corner of Mammon and Croesus. Both refer to his wealth and avarice.