Gulpin it Down! (LAoPtS)

Plot
The group are enjoying lunch in a harbor-side city park. As Brock reveals that they are very close to Petalburg City, Ash adds that he is eager to battle Norman again. Max warns that his father is a strong Gym Leader, while May is excited about being able to eat their mom's cooking again. As May reaches for her sandwich, a round green Pokémon slides by and inhales it with a gulp, and does the same to everyone else. It turns around and swallows hard before burping up Pikachu's bowl. The group stands up in amazement, with Ash brings out his Pokédex which identifies the menace as a Gulpin, a Pokémon with the ability to digest anything it swallows. Max is upset that it ate their entire lunch, but May thinks it's cute. Gulpin slides away as an emergency siren sounds, with Officer Jenny providing the voice over for an evacuation of the area.

The group walk along one of the deserted streets in the evacuated city, when a piercing scream breaks the eerie silence. They rush into a local bakery to see a woman cowering as a horde of Gulpin chow down on her loaves of bread. Ash orders Pikachu to attack, but the Gulpin quickly smother him in a dog-pile. As Ash finally recovers Pikachu, more Gulpin pour into the room through the ceiling air vents. The group, along with the woman, are soon nearly smothered by the horde of Gulpin. A sharp whistle from outside gets Brock's attention, who recognizes it as Officer Jenny's police whistle before he gets dragged down into the mob. Officer Jenny then appears in front of the bakery, commanding her Tangela to use Vine Whip to drag everyone to safety. The Gulpin then slide out as one mass into the street, leaving the bakery behind for a new food source.

Ash thanks Jenny for rescuing them, and Jenny warns them that it is too dangerous for them to remain in the city. The bakery owner promises to leave, but May and Max inform Jenny that they need a place to rest. Jenny asks that they follow her to the local Pokémon Center. When they arrive, they find Nurse Joy manning a control panel that is monitoring the city, confirming that the evacuation was complete and that measures to remove the Gulpin from the city are about to be launched. Ash comments that he had never seen a Pokémon Center like this, and Joy tells him that it had been specially outfitted to be a center for defense against Gulpin attacks. Professor Jacuzzi, a specialist on Gulpin research enters the room, declaring that the Gulpin population is once again on the rise. Officer Jenny explains that Gulpin attack the town around this time every year, devouring all of the food in the city, and that this year, the defense center's goal is to rid the town of Gulpin for good. Jacuzzi points out on a map where he knows the Gulpin are headed: the city's main food storage, in the center of town. He tells the group that he has been working on something that will "stop the Gulpin right in their tracks". Jenny then picks up an alarm on the system that shows the crowd of Gulpin moving toward the city's center. She calls for the launch of the "immobilization" maneuver. Ash asks if there is some way he can help.

Soon, Jenny and the group have positioned themselves at the end of a bridge leading toward the city center, with a large, armored vehicle parked behind them. They watch as the Gulpin mob quickly approaches. Jacuzzi, inside the vehicle, launches a missile at the mob. The missile breaks apart in mid-air, revealing three massive Pokéblocks. The Professor emerges from the armored vehicle to explain the giant Pokéblocks as being his own special blend, developed over several years, and are composed of all 108 ingredients which attract Gulpin. As the Gulpin leap to devour it, Jacuzzi activates a helicopter rotor on the device, which spins and carries the device up and away from the Gulpin. The Gulpin pursue the giant Pokéblocks, but the celebration is short lived as Team Rocket snatches the device with a giant mechanical hand attached to their balloon. The trio claim the device as theirs, but the Gulpin are set on getting the bait so they launch Sludge at the balloon. This quickly coats the balloon in brown gunk and weighs it down, causing it to crash. Jessie, indignant, stands up holding the Pokéblock device over her head, while the rest of Team Rocket gets ready to fight the Gulpin. The Gulpin respond by using Stockpile, before launching a group Spit Up attack that sends Team Rocket blasting off.

Brock leaps to catch the device with some difficulty while Jacuzzi commends the Stockpile and Spit Up combination attack. Max stops him to point out that the Gulpin are stampeding towards Brock, who is still holding the device. Jenny calls out her Tangela, Meganium, and Parasect and orders them to use Vine Whip, Razor Leaf, and Stun Spore, respectively. The attacks knock the Gulpin mob back, but Max is worried that the Poison types' resistance to Grass attacks will give them the advantage. Ash sends out his Treecko to help; the Gulpin launch several Sludge attacks at Treecko, who leaps to dodge them all. Ash commands Treecko to use Bullet Seed, which knocks the Gulpin back again. Jacuzzi then activates a rocket on the device, almost burning Brock in the process, which sends the device flying off into the distance. The Gulpin mob quickly follows, leaping into the river to give chase. The group watches the Gulpin leave, with Ash and May feeling like the worst is over. Jenny, however, recommends that they get back to the defense center. Later that evening, Team Rocket is wandering around the outskirts of the city, complaining of hunger. They happen upon the Pokéblock device, now stripped bare, laying on the bank of the river. In the distance, they see the mob of Gulpin headed back toward the city. Meowth recommends that they follow the Gulpin, which will most probably lead them towards food.

That night, a second alarm goes off in the city, and lights go off across the cityscape, blanketing the entire area in darkness. At the defense center, Joy turns on the emergency power, blaming the blackout on a severed underground cable. The monitor shows the Gulpin are headed directly for the storehouse in the center of town. Joy promptly closes a number of gates in the sewer, forcing the Gulpin to take a detour and head above ground. Jenny asks Jacuzzi to use another Pokéblock device, but the Professor says that the Gulpin are too smart to fall for the same trick twice. He tells Jenny to do what she can to hold back the Gulpin. Ash and Pikachu get ready to help the Professor again. Back outside on a deserted avenue, one of the manhole covers suddenly blows off the pavement, and the Gulpin horde quickly pour out onto the street, headed toward the large Poké Ball-shaped food storage warehouse. Ash, his friends, and Officer Jenny stand in the way. Pikachu leaps down from Ash's shoulder, while Jenny calls out her Pokémon and orders them to attack the Gulpin. Pikachu uses Thunderbolt, knocking out a number of the Poison Pokémon, but with every Gulpin that does down, more continue to press on forward. As the Gulpin inch on ever closer, Jacuzzi comes to the rescue with his armored vehicle, parking it between the group and the Gulpin.

Announcing that he is going forth with "Plan B", Jacuzzi activates two satellite dishes resembling ray-guns, which he aims at the oncoming Gulpin. With a click of a button, the dishes fire pink beams which make the Gulpin seemingly disappear. The Professor next aims the large cannon, firing the Gulpin one by one into the distance. As the group gaze on in wonder, Jacuzzi introduces his invention as the Mach III Particle Cannon, which was inspired by Gulpin's Stockpile and Spit Up attacks.

Soon Jacuzzi has one Gulpin left to contend with. However, before he can absorb it, the Cannon shuts down. Jacuzzi tells the worried onlookers that the machine "froze up" due to the subatomic particles becoming unstable. The lone Gulpin attempts to escape, but Ash sends out Treecko and has it use Bullet Seed. The Gulpin counters with Sludge. While the two Pokémon battle it out, Jacuzzi hurriedly tries to get the Cannon working again, and finally succeeds as the lights come back on. Ash orders Treecko to use Pound; just as Treecko's tail comes down on the Gulpin's head, Jacuzzi fires the particle beam, striking both Pokémon at once and absorbing them together. Inside the computer’s database, Gulpin responds to Treecko's attack by using Spit Up, knocking Treecko away and overloading the system. The machine begins to spark and smoke before the vehicle self-destructs. When the smoke clears, Jenny finds the rest of the group amongst the rubble, all safe and accounted for. Jacuzzi, on top of the wreckage, points out something unusual happening in the distance as two balls of pink light rise up from the debris. They are the formerly-absorbed energies of Treecko and Gulpin, now freed from the computer and as they take shape, they expand to an enormous size, finally revealing a gargantuan Treecko and Gulpin, which stare each other down as the restored city lights illuminate them from below.

May wonders out loud if the two had just evolved, which Max sincerely doubts. Brock asks the Professor what had happened, to which he explains that the Gulpin's Spit Up attack had reacted with the subatomic particles in such a way that it caused the two Pokémon's physical proportions to increase dramatically. As they watch, the Gulpin turns around and starts to slide away, wreaking damage and throwing up a dust cloud as it does so. Ash orders Treecko to intervene, and Treecko responds by landing in front of the Gulpin, pressing its hands on the Gulpin's head to block its path. As the two giant Pokémon fight, Jenny and Joy help the remaining citizens to reach the shelters. Unable to keep Gulpin back, Treecko uses its force against it and tosses Gulpin into a building. In retaliation, Gulpin opens its mouth wide and fires several rounds of Sludge with a loud belching noise, causing numerous explosions to erupt all across the city. Jacuzzi looks on, wondering if it was a mistake to build the cannon. Treecko is knocked down, causing rubble to fall, which nearly squishes Ash. Standing up, Ash notices the lake on the edge of town. Using a loudspeaker, Ash orders Treecko to force the Gulpin close to the lake; Treecko follows through by lifting Gulpin up over its head, and tossing it towards the lake. Sent flying, the Gulpin prepares an attack in mid-air.

Down below, Team Rocket is making a break for it with a load of pilfered food from the storehouse. A massive shadow passes over them, and they stop to look up; just then, the Gulpin fires its Spit Up attack, striking Team Rocket with the beam. Team Rocket ends up blasting off again, dropping their loot as they are sent flying. The Gulpin splashes down, and Treecko jumps in after it, getting ambushed as the Gulpin suddenly surfaces right in front of it and knocks it down. Treecko stands back up, and the two have a second staring match as the group watches from a rescue boat. They notice Treecko breathing heavily and realize that it must be getting very tired. Max cries out as the Gulpin uses Stockpile to prepare for another Spit Up attack, and Treecko evades as the beam tears through the water and blasts a hole in the concrete wall. Treecko starts to glow in mid-air as the effects of the cannon's malfunction wear off, and it lands on the boat near the group, back to normal size and panting in exhaustion. Joy comments that the intensity of the battle Treecko just fought had "brought him down to size", while Ash recalls Treecko.

The recovered Gulpin, on the other hand, is still massive, and is heading back into the city. Jacuzzi admits defeat, having used up every available resource. But then, May suggests that they try to capture it with a Poké Ball, as it is still a Pokémon. Jacuzzi steps forth and tosses a Poké Ball at the Gulpin, and it enters inside, only to bust out. However, Joy offers Jacuzzi a Heavy Ball, which he tosses, and is able to successfully capture the Gulpin. As morning comes around, Brock asks Jacuzzi whether or not the Gulpin would have turned back to normal as a result of being captured. Assured that it should be back to its normal size, Jacuzzi calls out the Gulpin, only to find that it is still giant. Jenny screams at Jacuzzi to recall the Gulpin, and he does so. Jacuzzi assures them that with his 50 years worth of Gulpin knowledge, he should be able to find some way to return the Gulpin to its normal state. Assured of this, the group looks on as the sun rises.