Serena, a young girl from Vaniville Town, is learning how to ride her mother's Rhyhorn. Unfortunatly, she ends up getting thrown off, getting scratches on her face when she lands. She claims to hate the training soon after.
Meanwhile, Ash, Clemont, and Bonnie arrive at Professor Sycamore's lab with the injured Froakie. The professor claims that the Froakie belonged to a trainer, and that the trainer wished to give up Froakie. Ash and company later find out that Froakie doesn't listen to it's trainer during battle, and often runs away. This particular trainer is also just one of many other trainers that have returned the troublesome water Pokémon back to Sycamore. Ash and company soon meet Garchomp, a Pokémon that lives at the lab. Sycamore says that Garchomp "has a good heart".
Professor Sycamore later brings Ash, Clemont, and Bonnie to the greenhouse that is home to many Pokémon like Zigzagoon and Azurill. Sycamore explains that he is studying Mega Evolution, a new type of evolution available to many fully evolved Pokémon. Jessie, James, and Meowth hear Sycamore from the roof of the greenhouse, and hatch a scheme. They disguise as scientists and enter the lab. Garchomp, the healed Froakie and one of Sycamore's assistants confront Team Rocket. James latches a mechanism around Garchomp's neck, hoping to control Garchomp. However, Garchomp goes on a rampage, sending Team Rocket blasting off and causing the enraged Pokémon to escape the lab. Ash and friends follow Garchomp into the streets of Lumiose City. The city is put under a state of emergency, and Garchomp's rampage is being broadcasted live on television. Serena and her mother watch the scene unfold on the TV.
Garchomp flies up to Prism Tower. Clemont helps Ash access a staircase at the back of Prism Tower, but one of Garchomp's stray Hyper Beams seperate Clemont and Bonnie from Ash, Pikachu, and Froakie. Ash makes his way up Prism Tower, and attempts to negotiate with Garchomp. The enraged Pokémon flies up to the highest platform of Prism Tower, and Ash climbs up a ladder to meet Garchomp face to face. Ash once again attempts to talk to Garchomp, who breaks out of it's rage for a split second before unleashing it's fury again. Ash grabs Garchomp, allowing Pikachu to use Iron Tail and shatter the collar. Garchomp collapses in a tired state, and as Pikachu and Froakie are making their way to Ash, the platform under Pikachu crumbles. Ash dives after his partner, much to the dismay of Clemont, Bonnie, Serena, and Sycamore. Ash is grabbed mid-air by the Mega Blaziken he saw in the before, and is safety escorted to the ground by the Pokémon. Mega Blaziken jumps back to it's masked trainer, and the two disappear. Ash yells after Blaziken, thanking the Mega Pokémon for it's help.
The next day, Sycamore gives Ash his Kalos Pokédex. Ash is also greeted by Froakie, who wishes to come with him on his journey. Ash accepts Froakie into his team, earning him his first Pokémon in Kalos.
The episode ends with Serena feeding Rhyhorn and remembering a younger version of Ash that she believes is the same boy she saw on TV.
Serena sees Ash on television and recognizes him as someone she knows and her flashback reveals she knew Ash before he became a trainer.