After hearing Gym Leader Olympia’s ominous prediction, Champion Diantha visits our heroes just as Ash and Greninja are doing some special training. To find out more about the bond they share in battle, Diantha challenges them, and Ash jumps at the chance!
Diantha’s Gardevoir easily evades every hit…until Ash and Greninja begin to merge. Gardevoir has to Mega Evolve to keep up, and the bond between Ash and Greninja intensifies like never before—until they both pass out!
Diantha promises to learn more about this bond and how it might relate to Olympia’s prediction. After his battle with the Champion, Ash is even more fired up for the Kalos League!
Olympia speaks with Diantha at the Anistar Gym about her vision of the future crisis of Kalos. When Diantha asks about "Those deep green eyes", Olympia shows her an image of Zygarde, which Diantha recognizes. Elsewhere, Ash and his Greninja are stretching and racing each other, with Ash believing he could master the Ash-Greninja form if he "becomes" Greninja. They stay at the same speed until Greninja starts jumping from branch to branch, with Ash mimicking Greninja, but he falls from a branch and Greninja helps him stand up. Serena, Clemont and Bonnie are waiting for Ash and Greninja on a glade, with Bonnie worried about Greninja's and Ash's delay, with Clemont finding the idea to "become a Pokémon" absurd, and Serena wanting Ash to not overdo with that idea. Greninja reaches the group first and Ash, very tired, arrives last.
Then, Diantha arrives in a helicopter and jumps from it. Diantha greets Ash and his friends and admits that she is there to find them after she listened to Olympia's prophecy. Team Rocket is nearby, observing them. Diantha speaks about the Kalos Crisis and she shows an image of an island attacked by Team Flare's weapon. She mentions that there are other places in Kalos that have been disturbed as well, fearing this is the beginning of the Kalos crisis, which leads them to believe Zygarde will play a key role. She shows an image of Zygarde to the group and starts to explain its role. Team Rocket recognizes Zygarde as the Pokémon that Team Flare kidnapped.
Clemont remembers the prophecy and how Olympia says that they are involved somehow in the crisis. Diantha asks if something changed around them, and Bonnie comments about Ash's Greninja being amazing, and they explain the phenomenon to Diantha. Clemont remembered what Olympia said about them when Greninja was a Frogadier. Diantha asks Ash to show Greninja to her in a battle against her Gardevoir. Team Rocket start planning to steal Diantha's Gardevoir when it Mega Evolves. Diantha's notes the Battle Pulse Meter and asks about it, so Clemont explains how it works.
The battle starts, with Greninja trying to hit a few hits, with Gardevoir quickly dodging them. Diantha comments that Ash's Greninja is well trained, but she sees nothing remarkable about it. She decides to go on the attack with a Shadow Ball and Ash tells Greninja to use Double Team to dodge the attack. Diantha responds by having Gardevoir use another Shadow Ball sequence to destroy the clones. Ash says to Greninja to use a Water Shuriken sequence in return, but this is dodged by Gardevoir. Gardevoir uses Moonblast, which is dodged by Greninja who still suffers a hit from the shock wave.
As Greninja recovers, Ash thinks of an idea on how to get close to Gardevoir. He tells Greninja to use Water Shuriken, which is dodged and creates a vapor cloud. This distracts Gardevoir and obscures its vision, allowing Greninja to finally land a hit with Cut. Greninja transforms into its Ash-Greninja form. Diantha and Team Rocket are all surprised at seeing this unknown form. Ash tells Greninja to use Cut, which is blocked by Gardevoir's Shadow Ball but still manages to deal damage. Diantha tells Gardevoir to use Shadow Ball, which is blocked by Greninja's Water Shuriken. Then Greninja goes towards Gardevoir using Cut, which is almost dodged, but Gardevoir takes another hit. Ash tells Greninja to use Aerial Ace on Gardevoir, and with the pressure against Gardevoir piling on Diantha decides to Mega Evolve it and the attack is blocked.
Ash, feeling that the battle is really starting now, has Greninja use Water Shuriken, with Ash and Greninja moving in perfect sync while the attack is executed. The attack is blocked by Gardevoir's Shadow Ball, so Greninja uses Aerial Ace, which is blocked by Reflect. Greninja uses Double Team and later Cut. Gardevoir tries Reflect again, but the protection was destroyed by Greninja's attacks. The water veil around Greninja lifts and creates a giant Water Shuriken on Greninja's back. It uses its new Water Shuriken to attack Gardevoir who fires a Shadow Ball to block Greninja's attack, but this fails and Gardevoir is sent flying. As his opponent lies stunned, Ash is ready to defeat Gardevoir and tells Greninja to use Cut. However before Greninja can begin, the energy between them becomes too much and both Ash and Greninja faint. While Diantha is going to check if Ash is okay, Gardevoir is captured by Team Rocket, but Gardevoir is easily able to escape from Team Rocket's "Unbreakable Mega Ring" and uses Shadow Ball to blast them off. Ash and Greninja recover, and Diantha says goodbye to the group and then suggests that they keep researching Ash and Greninja's power, for Kalos', Ash's and his group's sake.
Meanwhile, Lysandre is shown Z2, and allows the final stage of the project to commence. As they prepare to leave, Ash tells Greninja that they have to become stronger in order to master their power.