During an outdoor book fair, Krusty finds out he has a daughter -- from a one-night stand, with a female soldier, who fought in the Gulf War -- but loses her trust after gambling away her violin to Fat Tony, prompting Homer and Krusty to retrieve it.
Homer and Bart fix things around the house using fireworks. They destroy Lisa's room on her birthday, so to help make it up to her, the family goes to a book fair, where Krusty is signing his new book. A little girl named Sophie is in line and she tells Krusty that she is his daughter. Krusty flashes back to his experiences during the Gulf War.
Sophie's mother now hates Krusty. Krusty tries spending time with Sophie at the beach, but they don't bond very well. He sees Homer interacting with his kids and gets some parenting advice from him.
Krusty gambles away Sophie's violin, something that means a lot to her, in a poker game with Fat Tony. She gets very upset with Krusty, so he (with Homer's help) try to steal back the violin. There is a big mafia summit at Fat Tony's place and Krusty manages to escape with the violin and a bunch of cash (although it was implied that he didn't even know that there was money in the real violin case when he took it), although not without inadvertently starting a gun fight amongst the various mafia members when they dropped most of the other cases (which contained Tommy guns) and had the contents discharge ammo. He wins back Sophie's love. Then Frankie the Squealer, Legs and Louie chase Homer for his role in the caper (and presumably for his role in ruining their summit) shooting at him, but Homer outruns them nevertheless.
Originally, Homer was the one who was surprised to have a long-lost daughter, but this was changed to Krusty, to try and keep the episode canon, and not too far from reality.
Some sources suggest Christopher Walken originally agreed appear as himself. However, he then decided to demand a lot more money, than the producers were willing to pay. Instead, Jay Mohr did the voice of Walken.
In the end credits, Jay Mohr is actually credited with the voice of Christopher Walken, the credit says "Jay Mohr as Christopher Walken". This is the first time this has ever been done.
The episode received negative reviews from critics. DVD Movie Guide gave the episode a negative review saying "After two pretty good shows, Season 12 encounters mediocrity with “Poppy”. At no point does the program become poor, but it just lacks many real laughs. Outside of some amusing book fair cameos, this one fails to deliver much zing, and it tends to drag."
One of the many complaints of the episodes was that the plot and comedy relied too much on the guest stars to take the episode, instead of having an actual plot.