Hardly taking a second to relish his latest gym victory, Ash quickly sets course for the next gym in Blackthorn City. It isn't long, though, before a more immediate challenge confronts his small party. As Ash and friends enter a clearing, they find large numbers of Vileplume and Bellossom preparing to battle. With the help of his Bayleef, Ash is able to settle the disturbance, but afterwards learns from a researcher working in the area that this is a common occurrence. The winds in this area, it seems, carry particulates from the Leaf and Sun Stones that cause Gloom to evolve into either Vileplume or Bellossom. Depending on the wind's direction, different amounts of either stone are scattered over the local Gloom, creating temporary surges in the populations of either of their possible evolutions. Since there is no foreseeable end to this natural state of flux, Ash and friends must end this constant fight for supremacy by convincing these Pokémon to live in harmony despite this trick of the wind that puts one and then the other group in the advantaged position.
While heading toward Blackthorn City for Ash's final Gym Badge, Ash and his friends come across three sick Oddish underneath a tree. Brock cures them with some Pokémon food. Ash wonders how the Oddish got sick in the first place, so Pikachu asks them. The three Oddish lead Ash and his friends over a hill where they find a plain meadow with a huge tree home to many Gloom, Vileplume, and Bellossom. Soon, the Vileplume and Bellossom start fighting. Pikachu fails to break up the fights, so Ash sends in his Bayleef who separates them with her Vine Whip. Ash tries to get the Pokémon to apologize to each other and make friends with each other, but they refuse.
Soon, Steven, a Grass Pokémon researcher comes running towards the trio and he checks on the Oddish. The group makes their way to Steven's cottage-style laboratory with a Pidgeot weather vane on top. Steven explains that the Grass types are attracted to the area because of the running river and clear access to sunlight, and have divided the area into various territories. Steven and Brock explain the concept of branched evolution, and Steven reveals that due to tiny fragments of Sun Stones and Leaf Stones in the wind, Gloom evolve into Vileplume when the wind is from the northeast, or into Bellossom when the wind is from the northwest. Steven theorizes that the trace amounts can accumulate enough to have the same effect as a full Evolution stone.
A northwest wind blows through, which causes a pair of Gloom evolve into Vileplume in front of Ash and friends' eyes. The newly evolved Vileplume rush to join the others, sparking another fight between the Vileplume and Bellossom. Ash and Steven put on face masks to protect themselves against Vileplume's Stun Spore. Ash again summons Bayleef, also wearing a face mask, this time trying a Razor Leaf followed by a divisive Vine Whip, effectively ending the conflict. Steven explains many of the Oddish have left the area due to all of the fighting. He takes Ash and his friends to the tree he says started the conflict. The old tree provides shade in the morning blocking the Bellossom's sun access, while in the afternoon the tree stops sunlight reaching the Vileplume causing the opposition to migrate into the other's territory. A wind blows from the northwest causing some of the sleeping Gloom to evolve into Bellossom. These new Bellossom join some others to form a war front against a group of Vileplume. Bayleef's Razor Leaf and Vine Whip combo again separates the warring factions. Misty spots a peaceful group of six, three Vileplume and three Bellossom standing together. She notes they are a good model of cooperation.
Meanwhile, Team Rocket are sitting on a yellow picnic blanket in the meadow area about to eat a sandwich lunch. This peaceful moment is interrupted by a wall of angry Bellossom and Vileplume, the fight quickly disappears. Though the cloud of Stun Spore paralyzes Jessie and James. The two request Meowth go retrieve some Salveyo weed. Meowth quickly spots Steven's laboratory in the distance and peers through an open window before finding the much needed Salveyo weed. Meowth gives Jessie and James their doses of the medicinal plant and the two emerge from their paralysis slightly enraged by the Vileplume and Bellossom for interrupting their lunch. Meowth pitches the idea that a fleet of Grass Pokémon would relax Giovanni because of their decorative nature. The trio quickly set their sights on catching all of the Grass types to please their boss.
Steven and the group return to the laboratory to discuss tactics to resolve the conflict. This over-the-table discussion is interrupted by a storm alert. Steven's computer reveals the storm will be a strong northeasterly, which will result in a drastic increase in Vileplume numbers. Steven states that despite the conditions favoring Vileplume, it may lead to peace. Though both Misty and Bayleef have sympathy for the Bellossom, noting a balance is the ideal. The predicted storm soon rolls in, with a northeasterly, then a northwesterly sweeps in, causing an equal number of Gloom to evolve into both Vileplume and Bellossom. The storm clears and the sun emerges, as do the two warring fronts.
Before the first attack could land its mark, bombs fall from the sky, startling the Pokémon. It is revealed to be Team Rocket, flying in their Meowth Balloon and wearing gas masks. James announces their scheme to capture all of the grass Pokémon. James and Meowth sarcastically say they will use a 'Vileplume of Doom' robot and a nuclear-powered 'Bellossom Wrangler' robot before revealing they are actually just using a giant sack. The two sides again collide, with Team Rocket in the middle of the conflict zone. Meowth is quickly sent flying, whilst both James and Jessie's oxygen hoses loosen and they are again affected by Stun Spore. Steven and the group watch on, hoping the two sides would exhaust their efforts to realize that fighting does not solve anything. The fighting stops, with both sides collapsing from exhaustion.
The same group of six cooperative Bellossom and Vileplume again appear. Meowth translates that the Bellossom desire to become Vileplume, and the Vileplume wanted to be Bellossom, revealing the Pokémon have an identity crisis and desire to evolve a particular way. Misty attempts to settle the group's discourse, getting Meowth to translate human to Pokémon language. Misty explains the concept of fate, that the Bellossom and the Vileplume were meant to evolve into their current forms. Brock also makes a point of their common origins as Oddish and Gloom. Meanwhile, Jessie and James manage to regain consciousness thanks to some leftover Salveyo weed and are then caught by Ash and friends attempting to stash some Vileplume and Bellossom into their giant bag by hand. In response, other Bellossom and Vileplume release a Stun Spore attack. Jessie and James call upon Victreebel and Arbok to assist, though the two are quickly overcome by the Stun Spore cloud. The Bellossom and Vileplume unite against a common enemy, surrounding and fighting Team Rocket before Ash's Bayleef sends the trio flying. All of the Pokémon know now that they should all just live together peacefully as friends, and they now dance and chant together in three large circles. With that problem solved, Ash, Misty, and Brock say goodbye to Steven and continue their journey towards Blackthorn City.