After performing CPR on a tased raccoon, Lisa discovers her calling to be veterinarian and becomes an intern at the local office, but learns a tough lesson when her neglected class hamster dies. Meanwhile, Marge moonlights as a crime scene cleaner for a little extra money, and the trauma of the gory scenes make her increasingly dead inside.
The Simpsons are on an Indoor Water Park, but seeing that one of the main attractions had a very long line, Bart decided to pull a prank, announcing that the pools are infested with fishes that can "swim up people's wieners", and the only cure in case that happened was to roll naked on the snow. The prank caused huge panic, as all men rushed outside while Bart enjoyed the ride.
In the middle of the clutter, a raccoon entered the park, but one of the employees shot the animal with a taser gun. Lisa decided to help the animal by performing CPR on him, which worked. She was proclaimed as a local hero. As she proved to be responsible with animals, Ms. Hoover gave her the responsibility to take care of Nibbles, the class pet, during spring break. Realizing that being a veterinarian is her new calling, she volunteered as an intern on Dr. Budgie's clinic.
Later, Marge was stuck in traffic, as the clean up team was not able to reach a car accident scene (as the were stuck in traffic). Chief Wiggum convinced Marge to clean the street, and she did it remarkably well. Wiggum also invited 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 decided to do another clean up job to get money for a new fan. Her new job was to clean the scene of a "suicide-murder". She did get enough money for a new fan, but the body parts and the blood ended up traumatizing her.
Meanwhile, Lisa was so thrilled by the emotion of saving animals that she has become more ignorant towards others, that's when Bart decided 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 was extremely depressed and ill, and Dr. Budgie said that the only solution would be to perform surgery. However, Nibbles died only after a few seconds of operation, making Lisa extremely sad, as she technically killed an animal by neglecting it.
Back home, Homer realized 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. The final scene of the episode shows Nibbles' funeral and a video memorial with clips of past episodes.