The time has come for Ash's Eterna Gym Battle with Gym Leader Gardenia, who's a Grass-type Pokémon expert. The battle will be 3-on-3, starting with Ash's Turtwig against Gardenia's Cherubi. The sun is out and that means Cherubi's Chlorophyll ability gives it a speed boost, so before long Ash recalls Turtwig and swaps in Staravia. Meanwhile, Team Rocket is digging into the Eterna Gym in hopes of stealing Pokémon, but they end up hitting a network of underground tunnels instead. There they meet an Underground explorer who convinces them to help him dig for sphere and other buried treasures—too bad he doesn't plan on sharing the loot!
Back on the surface, it's Ash's turn to use the weather to his advantage. He has Staravia dive at Cherubi, using the glare of the sun as a distraction, and Staravia wins this round. Next up is Gardenia's Turtwig: Ash has a strategy to neutralize her Turtwig's speed, but Turtwig still manages to knock Staravia out with Leech Seed.
Now it's Ash's Turtwig against Gardenia's Turtwig! These Pokémon have faced each other before and neither one wants to lose. Ash's Turtwig wins this round, but it's been weakened by a Leech Seed from Gardenia's Turtwig. Gardenia's Roserade is then able to defeat Ash's Turtwig with ease, but that just means Ash's Aipom is ready to go next! More of Ash's quick strategizing wins the final round, and now he's earned his Forest Badge!
Ash and his friends arrive at the Eterna Gym for Ash's second Sinnoh Gym battle. Gardenia arrives shortly after and they prepare for their official battle. Outside the Gym, Team Rocket plans to tunnel underneath to steal Pokémon, though it is something they have done before and James is left completely unenthusiastic about it.
The three-on-three Gym battle begins as Gardenia summons Cherubi as Ash sends out Turtwig. Turtwig makes the first move with Tackle, but Cherubi dodges effortlessly. Turtwig follows up with Razor Leaf, but Cherubi dodges that as well. Brock notices that this amazing speed is coming from the harsh sunlight enabling Cherubi's Ability, Chlorophyll. Cherubi fires an instant Solar Beam attack, which Turtwig jumps up to dodge. Thinking it will take time to fire another, Ash commands Turtwig to use Razor Leaf, but to their surprise, the harsh sunlight allows Cherubi to fire another instant Solar Beam, negating the Razor Leaf and hitting Turtwig. Cherubi then uses Magical Leaf, but Ash recalls Turtwig before the move can land, causing Gardenia to commend Ash on the smart decision. Underneath, Team Rocket continues digging as they emerge into a lit underground tunnel. A miner approaches and Team Rocket recites its motto. The miner reveals that there are treasures underneath, Shards and Spheres among them. He recruits Team Rocket to help find the treasure and split the profit.
Meanwhile, Ash sends out Staravia. Cherubi launches another Solar Beam, but Staravia dodges and uses Wing Attack. Cherubi dodges and readies another Solar Beam, but is unable to do an instant-fire due to the sun now being concealed by clouds. Cherubi switches to Magical Leaf, and Staravia flies straight up, eventually getting out of Magical Leaf's range. Ash then has Staravia descend slowly, which confuses everyone, before having him use a nosediving Wing Attack. Gardenia commands Cherubi to dodge, but the harsh sunlight returns and blinds Cherubi, rendering it vulnerable to Staravia's Wing Attack, which explains why Ash waited to counterstrike. Staravia then follows up with Quick Attack, followed by an Aerial Ace that knocks out Cherubi.
Gardenia calls Cherubi back, praising Ash's clever strategy of using the weather to his advantage. She then sends out Turtwig as her second Pokémon, and Staravia takes on an angry look, wanting redemption after losing to Turtwig before. Staravia uses Wing Attack, but Turtwig quickly dodges. Ash then orders Staravia to push forward with multiple Wing Attacks, but Turtwig retreats backwards to dodge all of them. Gardenia wonders why Ash insists on sticking with a move that isn't working, until she realizes too late that his real intention is to trap Turtwig into a tree. Staravia then hits the immobilized Turtwig with Aerial Ace, causing a good amount of damage. Staravia tries to follow up with Wing Attack, but Turtwig's Leech Seed stops him in his tracks and saps his energy, exposing him to Turtwig's Tackle, which knocks him out. Ash recalls Staravia and sends his Turtwig back out, who is more than ready for a rematch.
Ash's Turtwig uses Tackle, but Gardenia's Turtwig dodges and fires Leaf Storm. Ash's climbs into a tree, dodges and starts circling Gardenia's. Ash's tries to attack from behind with Bite, but Gardenia's beats him to the punch with Leech Seed, draining him of his energy just like before. Ash quickly orders his Turtwig to use Synthesis, right before Gardenia's Turtwig fires Leaf Storm, which Ash's quickly dodges by leaping into the sunlight, fully healing in the process. He then fires Razor Leaf, which hits Gardenia's Turtwig. Both Turtwig then rush toward each other with Tackle, causing a big impact. After the dust clears, Ash's Turtwig falters, but is able to remain standing as Gardenia's Turtwig faints. Gardenia recalls her Turtwig, and Dawn congratulates Turtwig on redeeming himself, although Brock notes that the Leech Seed is still in play, and may cause Turtwig trouble against Gardenia's last Pokémon.
Meanwhile, Team Rocket continues digging while the miner stands back. Eventually, they unearth a Red Sphere. The discovery gives Team Rocket renewed digging as they dig out many Spheres, the miner scooping them up. After digging the miner gives Team Rocket lunch. While they eat, the miner slips out with the Spheres. Team Rocket finishes and notices that he has the treasure, but they don't care, wanting to eat more. Gardenia summons Roserade as her final Pokémon. Turtwig goes in for a Tackle, but Roserade surreptitiously uses Grass Knot to trip Turtwig, before using Weather Ball, the sunlight doubling its power and turning it into a Fire-type move, to torch Turtwig and knock him out. Ash recalls Turtwig and sends out his last Pokémon, Aipom, who makes a showy entrance as if she were in a Pokémon Contest.
Aipom uses Double Team and runs toward Roserade, but Grass Knot eliminates all the duplicates. However, the real Aipom jumps over the knot and lands a Focus Punch. Roserade fires Magical Leaf, but Aipom easily dodges by jumping along through the trees. However, Roserade grabs Aipom with vines from its bouquets and throws her down, before using Flash to blind her. Aipom tries to use Focus Punch, but due to Roserade's Flash, she simply thrashes about while Roserade steps aside, before hitting her with Magical Leaf. Aipom gets back up, still blind and thrashing about. Ash tells Aipom to calm down and use her ears instead of her eyes. Roserade uses another Fire-type Weather Ball, but Aipom's hearing allows her to use Focus Punch to deflect the attack, and Roserade ends up being blasted by its own Weather Ball. Roserade uses another Magical Leaf, but Aipom regains her sight and uses Swift to not only block the attack, but also create a Swift ring around Roserade. Aipom uses Focus Punch to catalyze the Swift stars to converge, hammering Roserade relentlessly. Aipom quickly follows up with Focus Punch, which hits Roserade hard enough to break a tree, slam into the wall, and faint, which gives Ash the win. Ash hugs Aipom at triumphing on the opponent's home field. At sunset, Gardenia gives Ash the Forest Badge.