While two men relax on a beach, one points out that the ice in their cooler has already melted, when a blue pod marked "513" falls into the water. The newly-released experiment slams his tail into the ground, causing rocks to split and the ground to shake.
While working on his his ship, Gantu orders 625 to turn off the news, which is alerting them of the earthquakes. Reuben tells Gantu that he will be prepared in case of emergency because he has been stockpiling sandwiches. Dr. Hämsterviel sends a transmission to Gantu, exclaiming that instead of results, Gantu only gives disappointment. Hamsterviel tells his lackey to bring an experiment at once. Soon Gantu is alerted about Experiment 513, and he leaves prepared to catch him.
Pleakley attempts to prepare Lilo, Stitch, and Jumba for further quakes, telling them not to panic, but he soon begins panicking. Stitch calms him down by pouring water on him, and Pleakley presents "Earthquake Preparation Kits". David Kawena comes by the house to tell Nani that her boss has requested her to come to work, although it is her day off.
At the beach shack, Mr. Wong is happy that the milkshake machine is unharmed by the earthquakes, but Nani expresses her distaste in having to work on her day off. While Stitch is helping clean the shack, he rides on a scuba tank, which smashes into the shake machine. Stitch attempts to repair it, but it is irreparable. Stitch takes the orange orb from the top of the machine and tries to eat it. Lilo snatches it away and tells Stitch he must care for things he likes. Stitch then cuddles the orange ball as if it were a kitten. Nearby on the beach, Gantu questions a couple about any anomalies they've seen since the quakes. Lilo realizes the seismic activity is likely due to an experiment.
Back at her house, Lilo consults Jumba and the computer about the experiment. Jumba warns them that 513 will find the spot on the island that will cause the biggest earthquake, and that they should stop him before he goes below ground. He equips them with a gel pack that will immobilize the experiment in a sticky mess. Pleakley enters to perform an earthquake drill: duck, cover, scream.
Lilo and Stitch trail 513 but when they confront him, he smacks the ground, causing the duo to fall over. Gantu rushes the experiment, hoping to blast him with a net, but in his holster is a sandwich. 513 flees underground, much to Lilo and Stitch's chagrin. Gantu attempts to follow 513, but has to fight Stitch. Stitch is disarmed, making Gantu believe he has the upper hand, but Lilo tosses the sticky blaster and Stitch encases the giant shark-like alien in it.
They enter the caves and are closed in by falling rocks. They locate Experiment 513, who keeps evading Stitch's shots. He tosses them from the cave and causes more rocks to collapse, blocking Lilo and Stitch's way. Stitch quickly checks on his orange ball, to ensure it is intact.
Jumba modifies Nani's buggy to include an industrial drill on the front but notices Stitch's ball. He is perplexed that Stitch is caring for something. After Pleakley panics more, Lilo and Stitch return to the previous rocky location. Gantu has just freed himself from the goo, when he realizes his experiment containment pod is loaded with sandwiches. 626 drills into the side of the rock, unaware that Gantu is following behind them. They locate 513 the source of the shaking, but he takes Stitch's ball. Soon the ledge crumbles beneath the car. Stitch must decide between saving Lilo or his ball.
He is able to catch Lilo and corner 513, but before retrieving his ball, Gantu captures the alien in the containment orb. Stitch tries once to free his cousin, but is not fast enough. Gantu blasts the car to prevent it from moving and abandons them in the caverns. As more rocks fall, Stitch hot-wires the car, allowing it to start.
The two return to the earth's surface outside a gas station where Jumba and Pleakley are buying more supplies. Jumba is still unable to understand why Stitch cares for the orb so much, but again the island is shaking. He is able to determine that Experiment 513 has begun a chain reaction which will soon tear the island apart. If another crack can be created at a right angle to the first crack, the quakes will stop.
They quickly rush to Gantu's ship to rescue 513, who has just been placed in the transport chamber. Stitch fights Gantu, as Lilo randomly presses buttons. The countdown to teleportation begins, but Stitch causes Gantu to smash the chamber walls and land on the pad. Experiment 513 is able to escape, but Gantu is teleported instead, much to Hamsterviel's annoyance. Using items from the earthquake kit, Lilo ties up 513.
At the ideal location for the opposing crack, Experiment 513 refuses to help until Stitch offers the orange ball. Lilo names him Richter and after releasing him, he rushes underground to create the crack to save the island. Stitch digs into the ground to retrieve his cousin and gives him the ball, which he earned. Lilo then introduces him to his one true place: the beach shack as the new milkshake mixer.
- Moral: You have to take care of things that you like, or you won't have them anymore.
- Richter is named after Dr. Charles Richter of the Richter scale.
- When Nani covers Lilo's mouth to silence her, she licks Nani's hand (much to the latter's disgust), referencing a similar scene from the first film.
- Gantu revealed that he came from the eighth planet of the Keplok system.
- In one scene, as Jumba starts showing 513 on his computer, he is not visible at all, even though he should be standing right in front of the computer. After a brief cutaway to Lilo and Stitch, Jumba then appears in front of the computer as he should.
- When Richter rolls into a ball, his eyes are covered by his butt, but in the next shot, they're not.
- This is the only episode where the Ice Cream Man ate a pink ice-cream cone instead of his usual green ice-cream cone.
- Although the term "boojiboo" is usually used in the romantic sense meaning "loved one" in Tantalog, Stitch and Richter use said term to refer to the orange ball that they both regard as precious.
- In "Ace", Richter is erroneously referred to as 501, a number taken by Yin who as well appears in that episode.