"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 a 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 saying 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 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 accidentall 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 twinky 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.