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!