At Cynthia’s vacation home in Undella Town, Ash, Iris, and Cilan are doing some special training before the Pokémon World Tournament Junior Cup, but Dawn has decided she needs a break. She tells the others about a nearby deserted island where a special Onix lives, and suggests that they go check it out! Cynthia’s butler Jervis takes them to the island by boat, and drops them off with a promise to be back before dinner—and an ominous warning to be very careful on the island.
They soon discover why: a huge cave on the island is home to a large group of Onix, and the enormous Pokémon aren’t too happy about having visitors! They attack, and our heroes flee, getting separated in the process. Iris and Dawn emerge in a wooded area, and Piplup finds itself in a clearing with Ash and Cilan. The guys run into more angry Onix while trying to escape, but Meloetta steps in to drive them away—first trying to soothe them with its beautiful song, and when that doesn’t work, the Pokémon changes its Forme and leaps into battle!
Meanwhile, Dawn and Iris are experiencing their own bad luck with the Pokémon they meet in the woods. Iris is thrilled to encounter a Magnemite, since she’s never seen one before, but gets zapped when she tries to pet it. And Dawn has a similar run-in with an adorable but poisonous Foongus! The girls eventually find their way into a canyon and reunite with Ash, Cilan, and Piplup—along with the group of Onix chasing them!
Oshawott and Piplup team up to battle the Onix, since their Water-type attacks are super effective—and during the battle, Oshawott learns how to use the powerful move Hydro Pump! But even that isn’t enough against so many Onix, and Ash calls on Meloetta to try singing again. It doesn’t seem to be working, and the Onix are closing in—but then, on the cliff above, an Onix of a different color rears up and roars at the others. It’s the special Shiny Onix our heroes have been looking for! It also appears to be the boss of the island, as the other Onix back off and return to their cave.
When Jervis comes to pick them up, Ash and the others tell him what happened. Jervis suggests that the Shiny Onix helped them escape because it was touched by Meloetta’s singing. As they head back to Undella Town, a beautiful rainbow inspires Meloetta to sing again, and everyone—Iris, Dawn, Cilan, Ash, and even Jervis—comments that Meloetta’s song reminds them of home. It seems the special Pokémon’s voice possesses a mysterious power...
...a power that Team Rocket is looking to take advantage of, as Jessie, James, and Meowth work on the next step in their villainous plan to capture Meloetta for the Boss!