Stitch is temporarily left deaf by an experiment that emits loud blasts of sound, leaving Pleakley (who has no visible ears) to attempt to retrieve said experiment.
Two tourists on their honeymoon get off a bus and approach Mrs. Lynne Hasagawa's fruit stand. They ask for some passion fruit and Hasagawa leaves her hose running in a pineapple basket and goes to check inside, but the hose drops water on an experiment pod, and 613 activates right in front of the tourists, blasting his horn and causing everyone nearby to temporarily lose their hearing.
The students at Moses' hula class are presenting ideas to improve the school. Mertle suggests painting a mural, but Lilo's idea involves installing an alien invasion alarm on the school's roof for the good of all humankind, and also for "any friendly aliens who may or may not be living there disguised as her dog." Needless to say, Mertle's idea is picked. Moses can tell Lilo is disappointed and advises her that if she really thinks the town needs an alarm, she can always talk to the mayor. Lilo is unconvinced because she thinks she is too young and small for the mayor to listen to her.
Back at the house, Lilo and Stitch hear 613's horn sound. Stitch uses his super-hearing to determine that Gantu is currently chasing an experiment, and the two run toward town. 613 terrorizes a television store, using its loud blasts to break speakers and shatter glass. Lilo and Stitch arrive to see Gantu firing laser blasts at the renegade experiment. 613 blasts Stitch with his noise horn as Gantu chases him off once again.
Lilo discovers that Stitch has gone completely deaf and goes to Jumba for help. Jumba identifies 613 as an experiment capable of creating sonic booms that cause acute hearing loss, but luckily, the effect is temporary. Jumba says that Stitch's super hearing made him super-sensitive to the attack; he needs to stay home and not try to catch 613. Lilo sighs as she realizes that unless she can find an experiment hunter with no ears, Gantu is sure to capture the experiment.
Luckily, an experiment hunter with no ears happens to walk in — Pleakley. It turns out that he actually "hears" using the antenna on his head, which is called a Plorgonarian "quart flanch", and is completely immune to the effects of 613. Jumba and Lilo run Pleakley through a hastily-constructed "boot camp" to prepare him for the task despite his protests. After a pitiful performance, Jumba resorts to giving him cybernetic enhancements. While Jumba is doing this, Lilo begins writing a letter to the mayor about her alien invasion alarm idea. She discards her draft and declares it's no use because the mayor would never listen to a kid.
Pleakley is worried about his ability to catch 613, but after encouragement from the rest of the family, he tracks the experiment to a nearby lava tube while the rest of the gang come up with a plan to distract Gantu. Stitch puts a sea shell into his mouth and blows through it, pretending to be 613 as Gantu is sent off on a wild goose chase across the island. Pleakley finds 613 in the cave and blasts him with a vacuum-powered containment hose, but fails to capture the experiment since the Plorgonarian had the nozzle set to reverse. 613 makes a massive noise and causes a cave-in, leaving Lilo and Jumba wondering whether Pleakley is okay. Pleakley dodges falling rocks and stalactites and saves 613 from being crushed, causing him to join Pleakley willingly out of gratitude.
Pleakley emerges from the lava tube with 613 in tow, to Lilo and Jumba's celebration. Pleakley names 613 "Yaarp" and Lilo radios Stitch of their success. Unfortunately, it is Gantu who answers, having captured Stitch moments before. He arranges a hostage trade at the local museum — 626 in exchange for 613. Lilo and Gantu both arrive and the deal seems to be solid, but after the experiments are swapped, Lilo signals to Yaarp to blow his horn at Gantu, stunning him as Yaarp makes his escape. Gantu blows a barrage of laser fire at Stitch, but he uses a mirror to reflect the beam back at the gun, destroying it. Museum security swarm Gantu believing him to be a burglar, so he is forced to flee by crashing through a wall.
The mayor of the town arrives and asks security what the suspect looked like, but the only description he gets is "a giant whale." Lilo takes the opportunity to tell the mayor that the "whale" is actually a space alien, and the mayor is actually relieved because an alien makes a lot more sense than a giant whale. Lilo gives the mayor her proposal for alien invasion alarms equipped on important buildings throughout the town. He actually goes through with the plan and installs an extraterrestrial alarm system at the hula school.note Yaarp has found his one true place as well — blowing his horn to make the alarm even louder.
The experiment of the episode:
- Yaarp (X-613): A four-armed, turquoise/blue lemur-like experiment who is designed to emit from the horn on his head powerful sonic blasts that can shatter windows, flatten buildings, and cause acute loss of hearing.
Tropes featured in this episode include:
- An Aesop: No matter how small you may be, you can make your voice heard and make a difference.
- Androcles' Lion: Pleakley saves Yaarp from death. Yaarp rewards him with kisses and the right to name him.
- Chekhov's Skill: Pleakley questions if step-aerobics count for military training and fighting skills. It's these skills that help him save Yaarp from the falling rocks and stalactites.
- Rule of Three: It takes Jumba three times to get Stitch to hear "Acute loss of hearing".
- Unlikely Hero: Pleakley unaffected by Yaarp's sonic blasts means he's the only one who can capture him.
- Weapons That Suck: Pleakley is supposed to use a vacuum device to suck Yaarp up. Unfortunately, he had it set to reverse.
