A truck is stopped on a road along the Kauaʻi shoreline. As its driver pours coolant into the overheated engine, some of the coolant spills into a crack, and suddenly the ground beneath him heats up. The man runs off in terror, the ground bursts open, and a strange lizard-like creature with lava spouts on its back crawls out of the hole. Meanwhile in Kokaua Town, an old woman relaxes in a hot tub when something falls into the water. The woman screams and runs off as an octopus-like creature appears and sucks up all the water in the tub.
Gantu comes out of the shower to find that two experiments, 501 and 502, have activated. However, upon seeing 625 napping, Gantu wakes his partner up and forces him to assist in getting the experiments. 625 reluctantly agrees but convinces Gantu that they should each go after one of the experiments instead.
Lilo and Stitch are visiting David's for a day of surfing. However, they suddenly fight when Lilo tries to get Stitch to help wax their surfboard, only for him to eat the bar of wax. David tries to get them to cooperate by telling them that they should work together instead of against each other, like yin and yang, two different things that come together to make a good thing, like how lava from the volcano and water from the ocean come together to make an island. Sadly though, it doesn't come through to our duo, and Stitch finishes off what is apparently the last bar of wax. "This is not harmony," says David.
Lilo and Stitch continue to argue until they find a lava-spewing experiment (the same one that viewers saw earlier) causing chaos in Kokaua Town. Lilo tries to come up with a plan, but Stitch rushes to grab the experiment and gets blown away. He tries to blame Lilo for his mistake, but then he gets blasted with water by another experiment (the same octopus-like experiment viewers also saw earlier). He ties up the experiment's tentacles, but Lilo protests saying that Stitch is hurting the experiment. Said experiment blasts both of them away, causing the duo to be almost washed away down a drainage pipe (Stitch saved both himself and Lilo).
The duo return home where Jumba tells them that they are dealing with Experiments 501 and 502, who are respectively capable of firing high-pressure water and hot lava. He also says that 501 and 502 are to never meet, for if they do so, their abilities combined can be explosive. Lilo and Stitch still continue to fight; they both proclaim that they can catch a experiment without the other's help and run off. Jumba is amused by Lilo thinking that she could capture an experiment without Stitch, considering Stitch's skills and abilities. However, Pleakley interjects saying that Stitch is nothing without Lilo and her Earth brains and instincts. The aliens make a wager in which whoever's selected experiment hunter gets an experiment first, the loser must perform the "Funky Glognar", a long and humiliating dance that is infamous throughout the galaxy.
As Gantu tries and fails to capture 502, not helped by 625's reluctance in putting any effort in experiment hunting, Jumba and Pleakley respectively have Stitch and Lilo gear up to catch the experiments, with Stitch going after 502 and Lilo aiming to get 501. But just when they were close to getting the experiments, their fighting cause the four to get stuck in Jumba's anti-heat foam he gave Stitch, allowing Gantu to easily come in and take the experiments right in front of them.
Stitch breaks everyone free from the foam and it becomes a mad dash for the experiments. However, Gantu falls for an egg salad sandwich trap laid by 625 and gets caught, causing 501 and 502 to break free. Gantu yells at his partner through a containment capsule that if they worked together in the first place, they wouldn't be in this mess, and Lilo says that the "big dummy" is right. She says that this is what David was trying to tell them about yin and yang. Thus, Lilo and Stitch finally reunite and work together again, using each other's tools provided to them to capture both experiments at the same time.
Unfortunately for Jumba and Pleakley, this also means they tied in their wager, which means that according to official United Galactic Federation wagering rules, they both lose and end up perform the embarrassing "Funky Glognar". Lilo, inspired by yin and yang, then decides that 501 and 502—now respectively named Yin and Yang—would work together in their "one true place"; using water and lava to create a whole new island.
Experiments who appeared or were mentioned:
- Yin (X-501): One of the episode's two Monsters of the Week; a teal octopus/squid-like experiment who sprays water at high pressures.
- Yang (X-502): The other Monster of the Week; an orange-red weasel/lizard-like experiment who shoots lava.
- Reuben (X-625): Gantu is displeased with 625 not pulling his weight and forces him to help capture the experiments. However, Reuben uses Gantu's arrogance to get away with being left alone.
Tropes featured in this episode include:
- An Aesop: We accomplish more working together than working against each other.
- Bread, Eggs, Milk, Squick: When Lilo explains to Jumba about David telling her and Stitch about how yin and yang being two different things coming together to make one good thing, like peanut butter and jelly or doughnuts and... mustard.
- Cassandra Truth: A man who gets singed and his car destroyed by Yang tries to tell a police officer about the lava-spewing creature, but the officer doesn't believe him and tells him to his move his car.
- Caught in a Snare: Gantu falls for a trap laid out by Reuben that was intended for either Yin or Yang. Gantu finds an egg salad lying on the ground and grabs it, causing him to fall into a hole in the ground that suddenly appears and gets his head stuck in a containment capsule that falls from above. Not only that, Gantu also loses the capsules that he put the experiments in, which break and free them.
- Extreme Omnivore: Stitch eats surfboard wax when Lilo tries to get him to wax their surfboard, which angers her and starts up their arguments with each other throughout the episode. Later on, he tries to eat Jumba's laser shield, which zaps Stitch and angers him for being inedible.
- Fire/Water Juxtaposition: Well, more like "Lava/Water Juxtaposition" in this case with experiments Yin (501) and Yang (502). The majority of the episode sets them up that they must never meet or the elemental clash would cause a deadly reaction, but it turns out Opposites Attract and their powers combined can create new landforms. Given that Lilo & Stitch takes place in Hawaii, this could very well be a reference to the legend of Pele and Naʻmaka in Hawaiian mythology.
- Foreshadowing: David telling Lilo and Stitch how lava and water come together to make an island.
- Funny Afro: Jumba wears a wig of one for the "Funky Glognar".
- Humiliating Wager: When Lilo and Stitch argue over who is better at catching experiments, Jumba and Pleakley get involved and make a bet whoever's protégé (Stitch for Jumba and Lilo for Pleakley) catches one of the two experiments first, the other must perform "the Funky Glognar, the witless dance of a thousand hours." However, Lilo and Stitch end up capturing both experiments together, which means both Jumba and Pleakley lose and have to perform the dance.
- Lady Looks Like a Dude: Yin is supposed to be a female experiment, but she's constantly referred to as a male by Lilo in this episode.
- Leeroy Jenkins: When Stitch first sees Yang, he almost immediately goes to grab the experiment, even though Lilo was trying to come up with a plan first.
- Meet Cute: Jumba warns that the Earth could be destroyed if Yin and Yang ever meet. They do, and the scene shown on the top of this page happens.
- Never My Fault: Stitch tries to blame Lilo for his mistake in rushing to grab Yang when she didn't do anything.
- Ship Tease: The way Yin and Yang look at each other when they Meet Cute strongly suggests this.
- Shout-Out: Reuben reads a book entitled Escape from Sandwich Mountain.
- Silly Song: The song of the "Funky Glognar" that Jumba and Pleakley perform near the end of the episode is just as ridiculous as their clothes and dancing.