The Lilypad Contest is coming up and Dawn wants Mamoswine to learn some Contest moves. But Mamoswine doesn't appreciate Pikachu and Piplup laughing as it tries to get things right! With its feelings hurt, Mamoswine charges off into the forest. Dawn follows it and the two of them are gone for the longest time. Ash and Brock are about to look for their missing friends when they're stopped by Verona, a forest ranger who forbids people from bringing Pokemon into the area.
Out in the forest, Dawn discovers the reason for Verona's rule: a Tangrowth swings in and uses Absorb on Mamoswine, then leaves a Sitrus Berry as its calling card before escaping. This Tangrowth drains the energy of any Pokemon that enters the forest--the bigger the Pokemon, the better, but it's just as happy to drain Meowth and Ash's Grotle too!
Once Mamoswine sees Tangrowth again, it tries to pick a fight, but Tangrowth doesn't want to battle. It's not until Verona arrives with Ash and Brock that everyone realizes what's happening: Tangrowth wants to save a dying tree that's a key part of the forest. Tangrowth absorbs Pokemon energy and leaves berries to help them recover, then uses Growth to heal the tree. Unfortunately, the tree is still dying!
Team Rocket shows up to vacuum up Tangrowth and the other wild Pokemon, but Dawn and Mamoswine take out Team Rocket with Mamoswine's new Hidden Power move. Once Team Rocket is gone, our heroes' Pokemon offer to help Tangrowth and Verona identifies the spot where all the tree's nutrients are stored. By using Growth on that spot, the tree is saved--and now Mamoswine has a great new move to use in the Contest!