While on their way to Violet City, Ash and his friends stop in the middle of a forest and notice that several of the trees are missing their leaves without showing any other signs of damage. Suddenly, Pikachu spots a horde of unknown Pokémon clamped onto a healthy tree trunk. Ash's Pokédex identifies them as Heracross and mentions their docile nature, great strength, and preferred food: tree sap. Thinking he can catch some new Pokémon and save the tree at the same time, Ash is about to throw a Poké Ball when a voice behind him tells him to stop. The trio turns around to see a man in a red uniform, who warns Ash that catching the Heracross will only cause more damage to the forest.
The man introduces himself as Woodruff, a forest ranger. After the group introduce themselves, Woodruff explains that the Heracross are important to the forest's natural balance. They share a symbiotic relationship with the local Butterfree population, who also feed on the tree sap but are too weak to break into the tree trunk to get to it. The more powerful Heracross dig into the trunk to start the sap flow, eat their fill, and then allow the Butterfree to come and feed. When Misty becomes worried about both the Heracross and Butterfree hurting the forest, Woodruff explains that the balance of nature in the forest was broken by invaders. Just then, a large herd of Pinsir stampede toward the tree. Woodruff identifies the Pinsir as the "invaders" as the Bug Pokémon leap onto the tree trunk and scare away the Butterfree. Ash yells at the Pinsir to leave the other Pokémon alone, while Misty demands that the Heracross do something about it. In unison, the Heracross fly up and attack the Pinsir, while Ash and Misty cheer them on. However, the Heracross are easily scared off by the Pinsir's massive pincers, and fly away. Woodruff tells the group that the Heracross are too peaceful to have any desire to take on the Pinsir; this worries Brock, who says that the forest will surely be destroyed if the Heracross don't do something to prevent it.
Ash notices a Butterfree still trapped in the branches of the tree. A single Heracross pops up in front of the Pinsir to defend the Butterfree, and is quickly attacked by their giant, spiked pincers. Ash, upset that Heracross is being ganged up on, sends out Bulbasaur to help it. One Vine Whip attack takes out all of the Pinsir, and Butterfree soon flies away, but Heracross is too tired to follow. The Pinsir regroup and ascend the tree trunk, which Ash thwarts by having Pikachu use Thunderbolt. Heracross looks on while Ash defends it, watching as Pikachu defeats the Pinsir with Thunder.
Once the Pinsir flee, Heracross flies down and approaches Bulbasaur and Pikachu. Ash thinks that it wants to thank his Pokémon, but instead, Heracross begins to suck forcefully on the opening of Bulbasaur's bulb, causing Bulbasaur some degree of pain as it drains his pollen. Bulbasaur knocks Heracross off with a strong Vine Whip, sending him sprawling on his back in the grass. While the group muses over whether or not Heracross was protecting the Butterfree or its food source, Woodruff is worried about the Pinsir returning. He explains that the Pinsir live on the opposite side of a river, and that something has been causing them to uncharacteristically cross the river and invade the Heracross' territory. Ash and Brock agree to help Woodruff save the forest, in spite of Misty's fear of Bug Pokémon. Woodruff begins to lead them toward the Pinsir territory.
While Bulbasaur clears the path ahead using Razor Leaf, Ash notices Heracross following them. Ash wonders if Heracross wants him to protect it, but Misty reminds him that Heracross liked Bulbasaur's nectar. Hearing this, Bulbasaur turns and slings a Razor Leaf at Heracross to try to scare it off. When Heracross looks up, confused, the group is now further down the path.
Once they are out of the woods, Ash recalls Bulbasaur. The group approaches a deep gorge that the river is running through. Woodruff points out a rope bridge that he uses to cross, only to find that the bridge is down. Looking into the gorge, Misty sees the Pinsir getting across by jumping along tree limbs. Ash accuses the Pinsir of having cut the bridge ropes, but when Woodruff inspects the severed ends of the rope, he points out that the cut is suspicious and that it could have only been done by a human. Angered by this, the group then ponders how to cross the gorge. They hear a sound, and turn to find Heracross pushing on a tree. Misty begins to fuss at it, but Brock cuts her off, realizing that Heracross is trying to push the tree over. The group jumps out of the way as the tree comes crashing down, landing neatly across the gorge. The group successfully crosses the river with it and enters the Pinsir forest, with Heracross still following at a distance.
As they enter the Pinsir territory, they notice a tree with a single gigantic Pinsir up in the branches. However, it turns out that it is one of Team Rocket's mechas, which is sucking out the tree sap with a hose. Team Rocket leaps out from the tree, recites their motto, and explains their plan to drain the forest of all of its sap and sell it. When Ash and the others berate Team Rocket for upsetting the balance of nature for selfish needs, Jessie and James to send out Lickitung and Victreebel, respectively, only for both Pokémon to become distracted by the tree sap. As a result, Meowth whips out a remote to the Pinsir mecha and commands it to attack Ash and his friends.
Ash sends out Bulbasaur and commands it to use Razor Leaf, but the robot negates the attack with its steel pincers. Bulbasaur then wraps it up in Vine Whip and sends it spinning, then Ash commands Pikachu to use Thunder Shock on it. However, the mecha absorbs the electricity and shoots it back at the group from its pincers, pinning them down. To everyone's surprise, Heracross intervenes and begins to push against the robot. At Meowth's command, the Pinsir mecha pushes Heracross for some distance. In response, Heracross gathers up its strength and begins pushing back, while Misty cheers it on, having changed her opinion of it. With their mecha getting beaten, Team Rocket fights over the controls, and the remote ends up getting destroyed in the scuffle. Unable to receive commands, the robot cannot fight back, allowing Ash and the others remove the sap tanks. Once that is accomplished, Heracross shoves its horn under the mecha and lifts it up onto its shoulders. With a mighty heave, Heracross tosses the mecha high up into the air. The lands on Team Rocket, then explodes, sending them blasting off.
Heracross strikes a victory pose as Ash and Misty congratulate it. They then notice a crowd of Pinsir approaching them from the surrounding forest, signaling that the Pinsir population has returned to its own territory, thus leaving the Heracross and Butterfree in peace. Back on the Heracross' side of the river, the group watches the Heracross and Butterfree feeding on a tree. The Heracross who battled Team Rocket's mecha approaches the group, and Ash and Misty commend Heracross' efforts and congratulate it on helping to save the forest. Saying goodbye to Heracross and Woodruff, they continue on their path to Violet City.
However, upon leaving the forest, Ash and his friends find out that Heracross is still following them. They realize that it wants to leave the forest behind and come with them. Heracross agrees, so Ash tosses a Poké Ball and it willfully enters. Ash celebrates his latest catch before the group continues on their journey.