During breakfast, Marge Simpson discovers a loose tile on the floor and says that she'll have to call a handyman. Homer Simpson gets the hint that he's not handy and decides to replace the tile himself. He succeeds with the help of an online tutorial, but soon realizes that he has trapped the family cat Snowball II inside the floor.
Meanwhile, Bart Simpson is participating of sled race on a hill, but Milhouse Van Houten is slowing them down, because he's scared. Bart knocks Milhouse off, but he loses control of the sled and hits the shopping cart of a homeless woman named Hettie Mae Boggs, throwing it and all of her things into a frozen river. Feeling guilty, he invites Hettie to the Simpsons' house.
Hettie is getting too comfortable in Bart's closet, so she decides to give Bart one dollar per day as a rent. Lisa Simpson soon realizes Bart's suspicious income, and discovers Bart's scheme, but Bart convinces her not to tell Marge, because if she does so, Hettie will get kicked out of the house. They also discover Hettie's incredible talent for music, so Lisa invites her to stay in her closet. During the recordings of one of her songs, Bart warns Lisa that she shouldn't be doing that, because if Hettie lets her down, Lisa won't be able to deal with her emotions.
Lisa showcases the song to Mr. Dewey Largo, her music teacher. He is initially hostile, saying he's come to hate music, but he is convinced to let her keep her idealism until she is taught by Mrs. Ortner the following year. She then seeks Mayor Quimby and Krusty the Clown where she manages to arrange a concert and a NPR interview for Hettie. However, during the interview, Hettie reveals that she's a drug and heroin addict. Lisa is shocked when she also discovers that Hettie could be very violent to the point of shooting someone (and possibly her parents) in the face. Meanwhile, Homer manages to free the cat from the walls, but he also traps Santa's Little Helper inside.
Later, Bart and Lisa come clean about Hettie to Marge, but Homer interrupts the conversation by falling through the roof trying to rescue Santa's Little Helper. Marge reveals that she has already rescued him. She also tells Homer about Hettie, but Hettie has gone missing with her concert in thirty minutes. Homer and Bart manage to find Hettie drinking on Cletus Spuckler's farm while Lisa is distracting the concert's audience by playing her saxophone.
After a conversation and almost getting shot in the face, Homer manages to bring Hettie to her concert, but Lisa had failed to entertain the crowd, so the place is almost empty. Lisa is heartbroken about Hettie, so she decides to sing one last song dedicated to Lisa, who decides to forgive her.
During the credits, Hettie is shown playing in a rehab clinic only for the interns to escape to Moe's Tavern through a secret tunnel. One of the interns, Disco Stu, comments that the bar is dirtier than the tunnel.