When Ash and friends reach Sandalstraw Town, they meet two familiar friends: Kenny and Barry! Kenny plans to enter the Sandalstaw Contest while Barry is training for the upcoming Pokemon Ping-Pong competition--but their conversation is cut short when Piplup glows as if it's ready to evolve! However, Piplup uses Bide to stop its Evolution and isn't well, so it's off to the Pokemon Center, so Piplup can rest. Dawn uses this opportunity to talk to Piplup about Evolution, but it gets upset and runs away!
Everyone splits up to search the nearby forest and Dawn finds Piplup using Bide to stop its Evolution. She doesn't understand why it won't evolve and that makes Piplup angry enough to run away from her again. When Dawn chases Piplup deeper into the forest, she finds it stuck in an Ariados web. Dawn saves Piplup from the Ariados, but then they're surrounded by an entire swarm of Ariados! Piplup uses Bide to defeat them and Dawn realizes this is just like the day she and Piplup first met.
Barry and Kenny are also searching the forest for Piplup, but Barry's Empoleon and Kenny's Prinplup are kidnapped by Team Rocket's giant Piprinoleon robot! The robot finds and grabs Piplup too, so Dawn sneaks inside the robot and confronts Team Rocket. Piplup forces its way through a duct in the robot to reach Dawn and back her up; it's wanted to remain a Piplup, so it can protect Dawn. Piplup uses Whirlpool to bust open the robot and Dawn's friends arrive to provide support. With their help, Piplup sends Team Rocket shooting into the sky and Dawn promises to always be partners with her Piplup. No Evolution needed!
Later, at the Pokemon Center, Nurse Joy gives Dawn an Everstone to prevent Piplup from evolving. Now everyone can concentrate on the Sandalstraw Contest--as well as practicing their Pokemon Ping-Pong!