Ash, Serena, Clemont, and Bonnie meet Diantha, the Champion of the Kalos region! Professor Sycamore is on the scene as well, doing research on Diantha's Gardevoir and its Mega Evolution.
Ash challenges the Champion to a battle, and under unusual circumstances, she accepts. However, their battle is quickly interrupted by the Team Rocket trio, who grab Gardevoir in a cage of energy and fly away! Thanks to the strong bond between the two of them, Diantha knows exactly where Gardevoir is, and she leads our heroes there at once.
She challenges Team Rocket, calling on the power of her Key Stone—and inside the cage, Gardevoir Mega Evolves! Professor Sycamore is delighted to witness a Mega Evolution in person, and Mega Gardevoir makes short work of the villains. Diantha has to get back to a movie shoot, but she agrees to battle Ash again someday...just as soon as he beats the Kalos League!
Ash and his friends notice a poster advertising an exhibition battle featuring the Kalos Pokémon Champion, Diantha. The others tell Ash, who has never heard of Diantha before, that Diantha is not only the strongest Trainer in Kalos, but she is also a famous actress. With that, Ash decides to request a battle with Diantha, and the friends run off to meet her.
Arriving at the stadium, Diantha's manager, Kathi Lee tells a large crowd that Diantha is not accepting meetings or interviews. Just as Ash and his friends are about to leave, Professor Sycamore appears and motions to them to follow him into a backroom. The Professor has come to examine the Mega Evolution that Diantha's Gardevoir possesses. The friends are excited to also hear that Diantha is in the room next door. Ash head towards the door, wanting to request a battle immediately, but before he can open it Diantha emerges, much to the awe of Serena, Clemont, and Bonnie. Professor Sycamore asks Diantha if she has thought about his request to leave her Key Stone temporarily in his possession, to which she respectfully declines, saying that the Key Stone is the very bond between her and Gardevoir. Professor Sycamore persists, however, and Diantha begins to offer a compromise for the next time she visits Lumiose City. Diantha's manager Kathi Lee, however, appears and abruptly tells Sycamore that Diantha's schedule is full. Diantha tells the group to feel free to watch the battle.
At the battlefield, the two opponents take to the field to a cheering crowd, Diantha and a previous year's champion from Brackish Town, Magnus. In the stands, Ash asks Professor Sycamore about the Key Stone. Professor Sycamore tells him the Key Stone is the stone Trainers hold, and the Mega Stone is the stone that Pokémon hold, both crucial to Mega Evolution. Diantha releases Gardevoir, who wears a Gardevoirite necklace. Professor Sycamore tells the group that a close bond between the Trainer and Pokemon is also crucial to Mega Evolution. Above, Team Rocket watches and decides to steal Gardevoir for themselves, hoping to please Giovanni. On the field, Magnus calls out Absol, and the battle begins. Magnus and Absol begin with Bite, but Gardevoir is too quick. Absol then uses Quick Attack, but again Gardevoir evades the attack, able to understand Diantha's commands merely through eye contact. Magnus's Absol uses Psycho Cut, but Gardevoir repels it with Shadow Ball, which rebounds to hit Absol. As a last resort, Absol uses Perish Song, but Gardevoir uses Moonblast to knock Absol out, winning the match.
Inside the stadium, a crowd again forms, hoping to see Diantha. An officer tells the crowd that Diantha has already left for a movie shooting. Serena suggests that the group go try Brackish Town's famous chocolate cake, Professor Sycamore opting to join them after he takes care of his data. However, when the group reaches the restaurant, they are greeted by a long queue. However, Bonnie and Serena are determined to get through the line, and manage to procure one slice. Faced with the predicament of sharing one slice between four people, Clemont pulls out a machine that can cut the piece into exactly equal pieces. Before he can cut into the dessert, the group notices a disappointed woman who was unable to procure a slice and they offer to share with her. The lady pulls down her glasses and reveals that she is indeed Diantha, and the group shares the dessert, after which Bonnie asks for Diantha to marry her brother. Ash determinedly asks Diantha to have a battle with him, and after his persistence, she accepts. Professor Sycamore appears, wanting to also watch the battle.
In a field, Ash begins to battle Diantha's Gardevoir with Pikachu. Ash begins with Pikachu's Iron Tail, followed up by Quick Attack, which Diantha's Gardevoir nimbly avoids. Ash next commands Pikachu to use Electro Ball and Thunderbolt, which Gardevoir also dodges. Gardevoir uses Shadow Ball, which sends Pikachu flying and Ash running to catch him. Seeing Ash's devotion to his Pokémon, Diantha recognizes his passion. Just then, Team Rocket releases a net, which captures Gardevoir in a transparent box. Diantha orders Gardevoir to use Shadow Ball on the box but to no avail. Jessie throws a smoke bomb and Team Rocket's balloon soars off. Diantha, who knows where Gardevoir has gone, leads the way.
Diantha recalls an experience in which she was lost in the fog while shooting in the Lapidarian Highlands, and her Key Stone led her to Gardevoir. Ash says he thinks their hearts were calling out to each other. The group reaches Gardevoir, and Diantha tells the others to stay back while she confronts Team Rocket. Diantha Mega Evolves Gardevoir into Mega Gardevoir, and this time, the force of Mega Gardevoir's Shadow Ball blasts open its prison. Diantha tells Mega Gardevoir to use Moonblast, which sends Team Rocket blasting off.
The group congratulates Diantha as Mega Gardevoir returns back to normal. A helicopter with Kathi Lee arrives to pick up Diantha. Diantha thanks Ash and the others, but has to go to her next movie shooting. Just before she leaves, Ash tells Diantha that he is challenging the Kalos League, and asks her to battle him when the time comes. Diantha tells him she'll look forward to it, and the friends and Professor Sycamore wave as the helicopter rides off into the sunset, having learned more about the secrets of Mega Evolution.