"Replaceable You" is the fourth episode of Season 23.
Bart and Martin invent a robot baby seal for a science fair. The plans turns into a success and the toy actually made Jasper and other senior citizens happy. However, the managers of some cemeteries realize that the number of funerals are depleting and turn the seals evil. Meanwhile, Homer gets a new assistant, but he becomes her assistant when he ditches work so he can watch a comedy film with Barney.
Bart is watching a Halloween special (parody of Frankenstein) of The Itchy & Scratchy Show when Lisa comes in with her science project. It proves that an asteroid will strike the Earth on July 15, 9789. Bart doesn't care claiming he will be president of Hell by then. Lisa says good luck with his project that she bets he hasn't started. Bart says his project is a fisteroid hitting the planet Nerd. Lisa says that is pathetic and Bart is annoyed because he actually did make a project like that. He later tries making a project called Cootie Shot. Milhouse asks him if it's ready but Bart says no as the science fair is a week away. Milhouse wants it because he and his dog really need a shot after they accidentally touched tongues. Bart asks how it's accidental after the fifth time and Milhouse runs out. Lisa thinks it is similar to the Cootie Patch he made one year earlier. Bart agrees it is rubbish and throws it in the bin. Bart is stuck saying ideas don't grow on trees. He looks out the window and observes Martin hung on a tree branch and gets an idea. He goes outside and slingshots Martin's underwear. Martin then agrees to help Bart if he is picked in basketball. He dreams about it and shouts "I'm in!" They walk past Milhouse who secretly licks his dog's tongue.
At the Power Plant, Homer is reading the Nuclear Safety Procedures. He reads: "In the event of a primary core breach you have 15 seconds to avoid a complete..." and yawns. He uses a key to open a 'top secret' draw and takes out a pillow. He puts it behind his head and slides down an eye mask saying 'Rock Star'. He pulls a blanket up and goes to sleep, snoring. A new woman arrives shouting howdy and wakes Homer up. She addresses herself as Roz Davis and says she is a girl from Ohio with a daddy with money. Homer takes her to where the workers drink coffee. He tells her to leave a little bit at the bottom so someone else has to make the next pot and Roz thinks it might be genius. Homer says old man Burns is coming, and then talks to him saying young man Burns. Mr. Burns leaves and Homer says "that sucker sucks up flattery like a twinkie sucks up gravy". Roz says nice comments to Homer and starts coughing and falls on the floor.
Meanwhile, at home, Bart is making a drawing of a Homer Humiliator. Martin says he doesn't see the scientific point in his invention. After seeing Santa's Little Helper wearing a Darth Vader mask, the boys decide to build a Robopet shaped like seals for everyone to cuddle with. At first, the first Robopet attacks Bart so Martin rewires it to make it friendly. To Lisa's dismay, Bart and Martin's project wins the science fair while she is left second place. Bart and Milhouse sell the Robopets to the Springfield Retirement Castle and all the old people love them.
Homer needs to leave work early and asks Roz to take his shift, to which she agrees. When Homer gets home, he sees Mr. Burns sitting on the couch next to Marge. Burns tells Homer that he found out him skipping work from Roz and demotes him to Roz's assistant. Fortunately, Homer learns from Ned Flanders that Roz freaks out when someone touches and tricks Mr. Burns into giving her a hug, causing Roz to attack Burns and get fired with Homer reinstated to his old job.
Meanwhile, the funeral directors dislike the old people getting happier and switches the wires on the Robopets to make them vicious again. When Alice Glick is killed by one of the Robopets, they ended up at Springfield Police Station. Bart and Martin ask Professor Frink to reprogram the Robopets from inside the police station to be friendly again. With the help from Benjamin, Doug and Gary, the build a machine that reprograms the Robopets to their harmless way and Chief Wiggum releases them for their cuteness. The Robopets are then sent back to the Springfield Retirement Castle much to the happiness of all the elderly citizens.