The Simpsons are visiting an indoor water park, but seeing that one of the main attractions had a very long line, Bart decides to pull a prank, announcing that the pools are infested with fish that can "swim up people's wieners", and the only cure in case that happened is to roll naked on the snow. The prank causes huge panic, as all the male visitors rush outside while Bart enjoys the ride.
In the middle of the clutter, a raccoon enters the park, but one of the employees shoots the animal with a taser gun. Lisa decides to help the animal by performing a CPR on him, which works. She is proclaimed as a local hero. As she proves to be responsible with animals, Miss Hoover gives her the responsibility to take care of Nibbles, the class' pet hamster during spring break. Realizing that being a veterinarian is her new calling, she volunteers as an intern on Dr. Lionel Budgie's clinic.
Later, Marge is stuck in traffic as the clean-up team is not able to reach a car accident scene (as they were stuck in traffic). Chief Wiggum convinces Marge to clean the street, and she does it remarkably well. Wiggum also invites Marge to make other crime scenes' clean ups, as they can keep all the money and jewelry they find. Back home, Marge realizes that their ceiling fan is old and unsafe, so she decides to do another clean up job to get money for a new fan. Her new job is to clean the scene of a "suicide-murder". She earns enough money for a new fan, but the body parts and the blood end up traumatizing her.
Meanwhile, Lisa is so thrilled by the emotion of saving animals that she has become more ignorant towards others. That is when Bart decides to remind her that she was so busy taking care of other people's pets that she forgot about her own class pet: Nibbles. He is extremely depressed and ill, and Dr. Budgie says that the only solution would be to perform a surgery. However, Nibbles dies only after a few seconds of operation making Lisa extremely sad as she technically killed an animal by neglecting it.
Back home, Homer realizes that if he took Marge to calm Lisa down about the deceased hamster, this could calm Lisa down, but also put Marge's emotions back together, and it does.
In the final scene, there is Nibbles's funeral and a video memorial with clips of past episodes.