Our heroes have arrived in Shalour City, but the Gym is inside the Tower of Mastery, which sits on an island with no apparent way to get there. Korrina comes to meet them and advises patience, which gives Ash some extra time to train! His Pokémon have really picked up on Tierno's rhythmic battle style, but it seems Ash doesn't quite have the dancing skills to pull it off.
Low tide reveals the path to the island, and our heroes cross, eager for a Gym battle—but first, Gurkinn has a history lesson for them. He tells how one of his ancestors discovered Mega Evolution along with his partner, Lucario, and mentions the family's treasured Scroll of Secrets...which catches Team Rocket's attention. The villains try to steal the scroll, but Korrina and her Mega Lucario quickly send them blasting off again!
As thanks, Gurkinn decides to let his granddaughter and our heroes see the Scroll of Secrets. Much to their dismay, the "secrets" turn out to be his own common-sense rules for life, rather than any new information about Mega Evolution. But there's no time for disappointment—it's time for Ash and Korrina's Gym battle!
Ash and his friends arrive at Shalour City for Ash's Gym battle against Korrina. At the Pokémon Center, Ash is excited, but Clemont and Serena are worried about Korrina's strength. The group rush outside and look out to the Tower of Mastery, also the location of the Shalour Gym, but Ash is left frustrated that the structure is surrounded by water at the moment. Soon, Korrina, who had been expecting them, and her Lucario show up. She comments that Ash must be fully prepared to battle the Gym. Ash is confident, though Serena remains worried that he hadn't yet come up with a strategy yet. Korrina then states that she has not lost to a single challenger since she last saw them. Korrina admits that it will be sometime before they can reach the Gym, much to Ash's frustration.
Clemont suggests that Ash could train more since Korrina has also become stronger. Ash agrees, calling out his Pokémon to begin training. Korrina suggests that she should leave, as watching would give away Ash's strategy. However, Ash is fine with Korrina staying, and Clemont takes out his gramophone so that Ash could start practising his 'moving and grooving' strategy. As they begin to dance, Ash's Pokemon, along with Chespin are dancing well, however, Ash seems to be having trouble coordinating himself. Clemont in response takes out his "Not-A-Very-Good-Dancer Automated Good Dancing Device" which he designed to help anyone to find their rhythm. The machine clamps around Ash and is switched on to full power. Being moved by the device, Ash begins to dance properly. Clemont increases the speed of the device, but this causes the machine to start spiralling out of control. Clemont attempts to stop it to no avail and the machine subsequently explodes. Next, Serena volunteers to teach Ash to dance. This continues into the afternoon, and despite the practice, Ash doesn't seem to have improved in his dancing skills. Clemont comments that maybe the rhythmic battling style isn't for Ash.
Korrina then calls out to Ash that soon they will be able to cross over to the Tower of Mastery. Waiting at the beach, the group watch as the tides part, revealing a path to the island. The group walk across in wonder, and Ash eagerly begins to rush into the Tower. However, Korrina's grandfather interrupts Ash, as he comes out to greet the group outside. In a dramatic voice, he announces that they must listen to the rules before entering the Tower of Mastery. He then invites them inside, and as the group enter, they marvel at the large statue of Mega Lucario inside the Tower. Gurkinn then begins to tell them a story concerning his ancestor and Mega Evolution. A young man with his Lucario, who had found two peculiar stones at this location. With these, they then achieved the world's first Mega Evolution. Ever since, Gurkinn's family have lived and guarded Mega Evolution island. Korrina exclaims that she wants to be as strong as her grandfather and her ancestor. Serena comments that she thinks Korrina is already strong, but Gurkinn interrupts, saying that she was not ready to be handed down the 'Scroll of Secrets' – a hidden family treasure which lists all one needs to know to be a Gym Leader. Ash's belly then rumbles, and Korrina suggests that the group have dinner and leave the Gym battle for tomorrow.
Meanwhile, Team Rocket are listening on, and plan to steal the Scroll of Secrets to hand it to their boss. While the others are sleeping, Team Rocket search inside Gurkinn's room for the scroll, but leave empty-handed. They then search the room where the others are sleeping, but also find no luck in finding the scroll there. Standing in front of the Mega Lucario statue, Team Rocket ponder about where the scroll is when Wobbuffet points towards the statue. Jessie then decides to search inside the statue. Using ropes, they climb the statue, and inspect inside Lucario's mouth. Meowth reaches inside the statue's mouth and finds the scroll, but as he takes it out, an alarm begins to ring.
Gurkinn runs out in a panic and sees Team Rocket with the scroll. In the meantime the others have also woken up. Gurkinn then begins to run after Team Rocket, but begins to fall behind and is left exhausted. The others instead run after Team Rocket. To block their path, James's Inkay uses Psybeam, destroying the path in front of them. Inkay begins to attack with Psybeam again, but is intercepted by Pikachu's Thunderbolt. Ash, Pikachu, Korrina and Lucario then resume running after Team Rocket. Outside, Team Rocket scurry to their waiting hot air balloon, only for Pikachu to destroy it with his Iron Tail. Jessie then begins battling by calling out her Pumpkaboo, though Korrina decides to take on Team Rocket by herself. Korrina's Lucario uses Bone Rush to deflect Pumpkaboo's Shadow Ball, and Ash exclaims that Korrina and Lucario are in perfect sync. Team Rocket order a triple assault, though Lucario dodges Pumpkaboo, Inkay and Meowth's attempts, and Korrina then decides to Mega Evolve Lucario. The Mega Evolution is successful, and Mega Lucario sends Team Rocket blasting off. Korrina manages to catch the stolen Scroll of Secrets and returns it to her grandfather. At the group's request, Gurkinn decides to read out the Scroll of Secrets since they had retrieved it. In an anticlimax, the scroll, rather than revealing deep secrets about Mega Evolution, reveals general ground rules written by Gurkinn.
The next day, the Gym battle between Korrina and Ash begins. Korrina makes a dramatic entrance with her roller skates and exclaims that they are finally battling.