While mailing a letter to her pen pal Henry, Lilo finds an experiment pod on the floor in the post office, numbered 509. According to Jumba, Experiment 509 is an agriculture experiment: designed to be an uncontrollable forest of destruction. Lilo is keen to activate it and make friends with it, but Jumba locks the pod away in a drawer, saying this Experiment is far too dangerous to risk activating.
At the Hula School, Lilo finds out that Mertle is entering an orchid in the upcoming Kokaua Town Fair, and makes a bet with her. Meanwhile, Stitch practices being a cowboy for the Kokaua Town Fair rodeo, and Pleakley plans to enter his giant "Pinormous" pineapple in the fruits and vegetables' competition. Because an orchid takes a long time to grow, and the fair is on the following day, Lilo decides to enter Experiment 509 in the competition. With Stitch's help, she steals 509's pod from Jumba's drawer and activates it.
The next day, Lilo, Stitch, and Pleakley arrive at the town fair with Pleakley's Pinormous and 509, who Lilo has named "Sprout". Stitch also meets his competition in the rodeo, five time champion Zach Mackillin. Lilo leaves Sprout on display with the other orchids entering the competition, and the accompanies Stitch to the rodeo. When Mertle approaches Sprout and notices he is not an orchid, Sprout lashed out at her, causing Mertle to accidentally break his pot open. Free from his containment, Sprout roots into the ground and grows into a massive forest of killer plants, which take over the Kokaua Town Fair.
In their struggle against the monstrous forests of Sprouts, Lilo and Stitch get to high ground at the top the pineapple water tower. Here, Lilo feels very ashamed of herself, as this was all her fault. Jumba warned her that 509 was too dangerous to activate, but she ignored him and let her desire to beat Mertle influence her decisions. Meanwhile, Nani and Jumba are watching the events on TV and are furious with Lilo for her recklessness. Just then, they spot the original Sprout, who was now a giant. Lilo deduces that all the other Sprouts must have spread from the original. Remembering what Pleakley said the other day, this gives Lilo and Stitch the idea to contain Sprout's roots so he will not be able to spread. Since he is now ginormous, they decide to use the water tower as container. Stitch then heads down and races through the forest of Sprouts on a giant mower, shredding them in his path. Eventually, and with some help from Zach Mackillin, Stitch manages to lasso the original Sprout, uproot him, and drag him into the water tower slated for demolition, trapping the latter and causing the forest to die off without the original Sprout. Lilo is unsure if this is Sprout's one true place, but it is the best she and Stitch could do on such short notice.
By sunset, the judges of the orchid competition reward Lilo with the blue ribbon, but decides to give it away to Mertle. Lilo has learned that a true champion either wins fair and square or loses fair and square. Nani then shows up and asks Lilo if she's alright. Lilo tells her she's okay and asks if she's grounded. Nani decides to ground her for just week, but promises her, she'll help her get a great plant for next year that will really beat Mertle.
- Moral: A true champion either wins fair and square or loses fair and square.
- This episode is an obvious parody of Audrey II from Little Shop of Horrors.
- Lilo is revealed to have a pen pal named Henry from Camp Pendleton.
- This is one of the rare times Jumba forbids an experiment pod to be activated, despite having an evil streak, the other time being in "Snafu".
- This episode shows that Stitch has a little common sense, as he was hesitant to steal 509's pod from Jumba's desk drawer, and later faulted Lilo for letting Sprout go out of control.