Ash and friends are leaving Mauville City, but Ash wants to go to the Pokémon Gym in Lavaridge Town while May wants to compete in a Pokémon Contest at Fallarbor Town. Max discovers the two towns are close to each other, so they decide on Fallarbor Town first.
Team Rocket stumbles into a huge watermelon patch but an Electrode disguised as a giant watermelon sends them blasting away. Some watermelons float downriver to be eaten by Ash and the gang. A Grovyle and Slugma attack them, and a girl named Natasha accuses them of being thieves. Her Electrode reveals the truth, so she invites them on a tour of her watermelon patch. After Natasha claims she's never lost a double battle, Ash challenges her.
Ash sends out his Treecko and Corphish to challenge Natasha's Pokémon, but they are quickly overmatched. Corphish and Slugma KO each other, leaving Treecko and Grovyle to duke it out. The battle is interrupted by Team Rocket returning to steal more watermelons.
Grovyle uses its Bullet Seed, but is gagged by an attack from Team Rocket's robot. Treecko uses Bullet Seed and manages to knock the melons down, and Pikachu zaps the robot, sending Team Rocket blasting off again.
Natasha and Ash resume the match, but Treecko is defeated by the power of Grovyle's Bullet Seed attack. The kids settle down to enjoy some juicy watermelon, and Ash and Pikachu try to encourage Treecko by spitting seeds at it. Treecko retaliates by using the real Bullet Seed move on them.
Ash and his friends leave the Mauville Pokemon Center for the next town. May declares they should head to Fallabor Town for her next Pokémon Contest, but Max consults his PokéNav and says the next Gym is in Lavaridge Town. As May and Ash momentarily argue on the next destination, Max consults the map on the PokéNav, and he declares they can do both as the two towns are neighbouring. Brock elaborates with the order, being Fallarbor Town, then Lavaridge. Meanwhile, Team Rocket, while hungrily wandering, happens to find a field of watermelons and eagerly steal some for themselves. Meowth then notices an unusually large watermelon before discovering that it is actually an Electrode. The watermelon-painted Electrode attacks Team Rocket with Rollout, causing them to be blasted into the sky, and the three watermelons they were holding descend and are carried down a river.
Down the river, Ash and company have just eaten Brock's prepared lunch. Max notices something floating down the river. Brock procures one of the watermelons from the river and experimentally taps it before affirming that it is good. Ash takes the watermelon from Brock and commands Pikachu to use Iron Tail to slice it open, though May hands it back to Brock for him to slice it. The group enjoy eating the watermelons, praising the sweetness. Ash spits out the seeds while eating, leading May to criticize his manners. May's reprimanding reminds him of the concept of seed-spitting, and he promptly brings out Treecko from its Poké Ball. Ash recalls his encounter with a Shiftry and asks Treecko to practice Bullet Seed with the watermelon's seeds. May comes over to Ash and asks about Bullet Seed, and Ash mentions how powerful Shiftry's Bullet Seed was. Ash demonstrates how to spit the seeds of the watermelon several times. Eventually, a small glowing ball of energy forms in Treecko's throat, but before Treecko can unleash it as Bullet Seed they are all attacked by a real Bullet Seed.
A Grovyle confronts the group, so Ash checks the Pokémon out on his Pokédex. A Slugma appears at Grovyle's side, and May checks the Pokédex entry for it in turn. Again As Grovyle and Slugma attack the group with Bullet Seed and Ember respectively, and the group attempts to escape. A girl's voice is heard, commanding the two attacking Pokémon to stop. A green-haired girl approaches and accuses the group of being watermelon thieves. May and Ash both attempt to explain their situation, but the green-haired girl points to the supposed evidence, namely the plate of watermelon slices. Spontaneously, the girl bites into one and spits out some seeds in their direction. The girl introduces herself as Natasha, the "Melon Master". Brock, Ash, and May attempt to explain the situation, though Natasha remains accusatory and demands they confess. As the watermelon-painted Electrode appears, May becomes frightened by the seemingly giant fruit, but Max informs her that it is actually an Electrode. Natasha points at them, stating that they must be the watermelon thieves that Electrode encountered before. Electrode shakes its head and, realizing her mistake, Natasha is apologetic. The group forgives her, adding that her watermelons were particularly sweet. Natasha decides that a tour of her watermelon patch is in order.
Natasha leads the others to her field, where she tells her Electrode to keep watch for the watermelon fields, and Electrode rolls away. Ash comments that Grovyle's Bullet Seed attack was really good and Max compliments Slugma's Ember as well. Natasha thanks their praise, and adds that the three of them have not lost a Double Battle yet. Ash states that he is skilled also, and made it to the best eight in the Silver Conference. On this, Natasha challenges Ash to a Double Battle match with her Grovyle and Slugma, and Ash eagerly accepts. Brock takes his position as referee, and states that the match will begin when both Trainers give the okay. Ash and Natasha both say such, and the battle commences. Ash sends out Treecko and Corphish. May, on the sidelines, asks why Ash would send out Treecko against Grovyle, its own evolved form. Brock deduces that this decision is for Treecko to gain some advanced training. Ash commands Treecko to use Pound against Grovyle, and Corphish to use Bubble Beam on Slugma. On Natasha’s orders Grovyle dodges and fires a Bullet Seed to neutralize the Bubble Beam. Natasha commands Grovyle to fire Bullet Seed on Treecko, and Slugma to use Ember on Corphish. Ash counters this by commanding Corphish to use Harden while Treecko hides behind it. Natasha momentarily compliments Ash's strategy, and Ash orders that Treecko use Quick Attack on Grovyle and Corphish hit Slugma with Crabhammer. Natasha has Grovyle dodge Treecko’s advance and it lashes Corphish with a Leaf Blade. Shortly after, Natasha commands Grovyle to use Bullet Seed and for Slugma to use Flamethrower. Equally quickly, Ash counters by commanding Corphish to use Harden and Treecko to be shielded behind Corphish again. The two attacks hit Corphish, who emerges from the barrage slightly injured. Concerned, Ash asks if Corphish can still battle. Although tired, Corphish affirms this, and Ash reconsiders Natasha's strength, seeing why she has never lost at double battling. Later, Team Rocket reappears in Natasha’s patch, and consider how to steal the melons without Electrode interfering. Meowth has a fantasy involving the usefulness of the watermelons to the boss. As the trio cheer for their promotions, Electrode in shock.
Ash and Natasha’s match continues. Ash commands Treecko and Corphsh to attack, but Grovyle again dodges and uses Leaf Blade on Corphish. Natasha then tells Slugma to use Body Slam and for Grovyle to use Bullet Seed. By this command, Grovyle and Slugma rush forward to attack, and again Ash tells Corphish to use Harden and Treecko to be shielded behind. Slugma jumps into the air and covers Corphish, negating Treecko's effective protection. With this protection negated, Grovyle leaps into the air and hits Treecko with a direct Bullet Seed. Ash asks Treecko if it can still keep going, and, with some exhaustion, Treecko manages to stand. Equally exhausted, Corphish also struggles. Max and May comment with awe on how strong the two of Natasha's Pokémon are, and Ash exclaims to Natasha that he will demonstrate a combination of his own. Ash has Treecko grab a hold of Corphish's right claw, and Corphish propels it forward with its Crabhammer. Treecko increases its velocity with a Quick Attack and it slams directly into Grovyle, while Corphish batters Slugma with Crabhammer. Natasha remains confident as she order Grovyle to use Solar Beam. Ash commands Treecko and Corphish to perform the shield maneuver again, and Grovyle launches Solar Beam. Corphish braces against it, although more exhausted than before. Natasha commands Slugma to use Flamethrower, and Ash has Corphish use Bubble Beam. The two attacks collide into a plume of steam, where both Slugma and Corphish faint from the attacks. As Natasha and Ash recall their fainted Pokémon, Electrode comes over to Natasha and informs the group that it is the watermelon thieves again, prompting her to leave with the others in pursuit.
Team Rocket is in the field, inside a watermelon-picking Cacnea-shaped mecha with a basket tied to its back. The mecha then confronts the group, Jessie and James on the top beginning the motto. James pulls a lever, causing the mecha to consume an armful of melons before launching the leftover seeds directly at the group. Natasha commands Grovyle to use Bullet Seed at them, and Ash tells Treecko that it can learn Bullet Seed by watching Grovyle. Grovyle then launches Bullet Seed at Team Rocket, hitting the mecha. Meowth pulls a lever, and the Cacnea mecha pulls out a bazooka and fires an X-shaped tape at Grovyle's mouth, preventing it from using Bullet Seed. With this, Ash orders Treecko to use Bullet Seed. At first Treecko does not succeed, but with Ash's encouragement Treecko's second try hits the rope of the mecha's watermelon basket, causing the stolen fruit to fall. Natasha yells for everyone to catch the melons, and their scurrying attempts helps rescue the fruit. Ash commands Pikachu to use Thunderbolt on Team Rocket, and the resulting detonation causes the mecha to explode and the evil trio are sent blasting off.
Returning to the battle, Ash commands Treecko to use Bullet Seed. Treecko attempts to perform Bullet Seed again, but it fails. Natasha commands Grovyle to use Leaf Blade, though Treecko dodges and Pounds it in a follow-up attack. Grovyle unleashes a Bullet Seed, but Treecko evades the move and knocks Groyle to the ground with a Quick Attack. Natasha asks Grovyle to get up, and with a struggle Grovyle does so. Treecko is panting heavily, and Ash notes that Treecko is almost out of energy. Natasha realizes that Grovyle does not have enough energy for Solar Beam, so she calls for another round of Bullet Seed. Ash retaliates, commanding Treecko to use Bullet Seed also. The Bullet Seed attacks collide in midair, but suddenly Treecko's Bullet Seed is depleted and Grovyle's Bullet Seed overwhelms Treecko. Brock declares that Treecko is unable to battle, handing Natasha the win.
After the battle, Natasha rewards everyone with some sliced watermelon. Treecko, however, is sitting by itself, clearly annoyed by its loss. Ash and Pikachu walk over to it, with Ash assuring Treecko that it will take time to perfect its Bullet Seed technique. Treecko dismisses Ash’s optimism, so Ash and Pikachu spraying seeds at the back of Treecko's head. Treecko, irritated, retaliates by firing a real Bullet Seed attack at them, and the two are knocked backwards. Brock comments that all Treecko needed was the right motivation, and Ash, offended, questions why Treecko was motivated to attack him. May responds that what Treecko learned was important, not what the target was.