Ash and the gang are practicing in a forest on their way to the Grand Festival. They come across what appears to be another Piplup, and then Pikachu...and then, one by one, all of their other Pokémon seem to appear in pairs! It turns out that a duo of transforming Ditto are mimicking their Pokémon!
The two Ditto are with Trainer Marina, a feisty girl who is devoted to them. But when Ash challenges her to a battle, Marina shows her inexperience. Brock explains that she needs to learn the moves of the other Pokémon that her Ditto transform into if she wants to battle successfully. Marina breaks down and tells them that her feisty nature is really a cover-up: she's usually very nice and gentle.
Brock offers to teach her some of the strategy about different types of Pokémon in battle. Just then, Team Rocket decides to steal Marina's Ditto duo! An intense battle ensues, and with Brock's help and advice, Marina gets her Ditto back...and Team Rocket is sent blasting off again!
As our heroes bid farewell to Marina, it appears that she has feelings for Brock, but he runs off to flirt with yet another girl! Marina decides being tough is still the way to go, and our heroes resume their journey to Lake Valor and the Grand Festival!