Expedition to Onix Island! (Japanese: イワークの島でサバイバル！Survival on the Island of Iwark!) is the 87th episode of the Best Wishes series, and the 744th episode of the Pokémon anime. It first aired in Japan on July 5, 2012 and in the United States on November 10, 2012.
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!
On the shores near Cynthia's villa, Ash, Iris, and Cilan are preparing for the upcoming Pokémon World Tournament Junior Cup tournament while Meloetta watches. Soon, Dawn comes down onto the beach to join them and suggests a trip to a deserted island to take a break from the training. Iris and Cilan both immediately agree, but it is not until Dawn informs him of a special Onix living there that Ash agrees to stop training. According to Dawn, it is rumored that people who are fortunate enough to meet this Onix will have incredible things happen to them.
Jervis agrees to take the group in his boat to the island, and Meloetta arrives at the last minute, saying that it wants to go as well. Just after they step off of the boat, Jervis warns them to be careful during their stay. Piplup and Oshawott soon make trouble with a Krabby and Kingler, much to everyone's amusement. While Cilan and Pansage prepare lunch, the others play in the water. Dawn deems Cilan's cooking to be good enough to rival Brock's, while Piplup and Oshawott soon resume their fight over who Meloetta likes more. The fight becomes so intense that Pikachu, Axew, and Pansage have to intervene. With lunch over, everybody decides to go find the special Onix.
Ash and his friends enter a cave, searching for some sign of an Onix. Iris and Cilan both show apprehension when Ash and Dawn explain how large they are, and one suddenly begins barreling through the cave. Sprinting away from it, they all encounter a few more Onix before escaping the cave - but unfortunately, they become separated in the process. Iris, Dawn, and Axew are together, but they realize that Piplup must be with Ash, Cilan, and the other Pokémon. After trying and failing to reenter the cave, Dawn and Iris decide to head for the beach in the hopes that Ash and Cilan will do the same. Meanwhile, Ash, Cilan, Meloetta, Piplup, and the other Pokémon begin trekking to the beach as well. Soon, however, they are accosted by three Onix. Although Piplup and Oshawott try to drive them away, they are unsuccessful, but Meloetta intervenes just in time. It uses Relic Song to change to its Pirouette Forme and defeat the Onix with Close Combat.
Iris and Dawn continue trying to find the beach, and they agree to follow Dawn's "hunch", although it results in a dead end. This time following Iris's suggestion of which path to take, they find themselves at a cliff. After Iris is attacked by a Magnemite and Dawn a Foongus, Iris comes up with the idea of swinging on vines to cover ground at a much faster pace. However, the vine snaps before they can get very far. As the two girls are being chased by yet another Onix, Ash and Cilan accidentally disturb no less than five sleeping Onix themselves and the two groups are chased into each other. Stuck between a pile of rocks and ten attacking Onix, Oshawott and Piplup begin to fight back. Hearing Meloetta's frightened cries, they manage to easily defeat several Onix each, and Oshawott learns Hydro Pump in the process. However, despite all of this, the Onix keep on coming, and even Meloetta's song cannot calm them down.
Suddenly, a Shiny Onix appears and orders all the other Onix away from Ash and his friends. On their way back to Cynthia's villa in Jervis's boat, Jervis surmises that Onix did this because it was touched by Meloetta's song. As Meloetta begins to sing again, they are all surprised by Jervis's assertion that Meloetta's voice contains mysterious power. Elsewhere, Team Rocket drops a sensor into the ocean, looking for undersea ruins and boasting to each other of their plan to capture Meloetta.