Mystery on a Deserted Island! (Japanese: お宝の謎！無人島アドベンチャー！！Mystery of the Treasure! Deserted Island Adventure!!) is the 135th episode of the Best Wishes series, and the 792nd episode of the Pokémon anime. It first aired in Japan on July 25, 2013 and in the United States on October 19, 2013.
When Alexa shows our heroes a mysterious treasure map linked to their next stop on Frond Island, everyone is very excited! She tells them the story of a pirate, his Pokémon (Shiftry, Nidoking, Mismagius, Honchkrow, and Vulpix), and the unknown treasure his ship contained. Jessie, James, and Meowth, who have been eavesdropping on our heroes’ conversation, decide to wait patiently until the Twerps find the treasure so they can steal it—and Pikachu too, of course...
Alexa takes Ash, Iris, and Cilan by boat to a small, deserted island nearby. After a narrow escape from a whirlpool, they find a hidden inlet that leads them through some tall cliffs as three Honchkrow keep a watchful eye on them. There, decaying on a beach, is a very old pirate ship! But Team Rocket has been following them underwater in a pedal-powered submarine, and when the villains surface too quickly, they ram the ancient ship by accident, sending it slithering back into the depths.
Ash is afraid the treasure went down with the ship, so he’s relieved when Alexa spies a door covered with symbols that resemble the ones on her treasure map! Cilan moves the symbols around to match the map, and the door opens to reveal a staircase.
At the top of the stairs is a dense forest, where our heroes are attacked by two Mismagius, then a Nidoking, and then three Shiftry! Alexa starts to put the pieces together and realizes that the Pokémon who have been attacking them match the story of the pirate and his crew! But they haven’t seen a Vulpix yet...
On the mountaintop, they find four Pokémon statues and one empty pillar. Then, the final Pokémon attacks—it’s a Ninetales, who must have evolved from the Vulpix they’ve been looking for! After they drive off the Ninetales, they discover the fifth statue and the secret entrance at its base. Sensing an opportunity, Jessie, James, and Meowth finally show themselves and prepare to battle our heroes for the treasure—but they’re all tired out from fighting the island’s Pokémon, and they’re quickly sent blasting off with nothing.
Sure enough, there’s a treasure chest inside the final statue, and it holds an amazing hoard of Evolution Stones! Our heroes and Alexa then realize the Pokémon that attacked them were protecting this treasure. Alexa decides to delete her records of this adventure, and everyone agrees to keep the island a secret, for the good of the Pokémon that live there. They return to the ship to continue their journey through the Decolore Islands and ultimately to Kanto!
As Alexa is looking at the Frond Island, Ash, Iris and Cilan meet up with her and she shows the group, a secret-coded treasure map and informs that she is going to find the treasure. As everyone is surprised, she explains how she once visited a pirate museum and saw a replica of a pirate ship as well as many on-boarded treasures the ship had. She adds that the pirate captain had many Pokémon and that the pirate left his treasure on one of the Decolore Islands. An excited Ash asks Alexa if she knows the kind of the treasure, but she replies in the negative. She, however, adds that after some research, she did manage to find this map, which has a mysterious secret code. She explains that the island where the treasure is hidden is on a deserted island near Frond Island, so she decided to go there and investigate. Cilan marvels at the excitement of treasure hunting and Ash and his friends, all hyped up, decide to go treasure hunting with Alexa. Team Rocket happens to be eavesdropping on the group's conversation and decide to find the treasure as well, along with Pikachu of course.
After getting off the ship, Alexa and the group heads to the deserted island on a motorboat. Alexa begins recording the expedition and Ash gazes at the long cliffs. Iris points to a passageway, stating that it might lead them inland. As they are about reach the inland,however, a whirlpool generates and the boat is sucked in.
Having braved the whirlpool, Ash wakes up, followed by Alexa, who is glad that they are safe and Iris and Cilan call the experience scary. In her documentary, Alexa notes this strange experience, emphasizing the fact that whirlpool might have been created to prevent them from getting in. Iris points that the passageway is like a maze. Suddenly, the cry of three Honchkrow scares Alexa's Helioptile. Iris notes the Honchkrow happen to be staring at them, but Cilan shrugs her off, stating that they are just evaluating the group. Underneath the boat in the waters, Team Rocket is following the group in their Basculin submarine. The group finally reaches the island shore and Ash points to a beached ship, which Iris claims to be the pirate ship. As Pikachu hits the ship's portside with his tail, a part of the portside breaks open and the group goes inside to investigate. Beneath the ship, Team Rocket has also reached the shore, but a sudden upwards movement of the water pushes their submarine to the ship, causing it to slam into the ship's base. Ash and his friends escape the shaking ship in time as it is sucked down in the water along with Team Rocket's submarine. Ash is disappointed, thinking that the group has lost their chance to discover the treasure but Pikachu points to a doorway. Alexa exclaims that the doorway might hold the treasure; for that the secret code must be deciphered. Cilan points out that none of the symbols on the map match those on the doorway. Ash puts his hand, causing a piece of the door's code to change position. Cilan then deciphers that the pattern on the door must be made similar to the pattern on the map to open it. Perched on a tree, a Honchkrow happens to watch all the commotion and then flies away.
With the code successfully deciphered, the doorway reduces to rubble and a staircase is discovered. The group reaches the top of the staircase where they are attacked by two Mismagius. Ash and Iris send out Snivy and Dragonite to battle and the group successfully evades the two Magical Pokémon. Out of blue, a Nidoking is seen attacking three Eevee who are rescued by Pikachu. Iris then wonders the reason for attacking the Eevee and Cilan suggests that it might be for food. He then gives Pokémon food to the Eevee while Iris gives fruits to them. Helioptile then climbs down Alexa to play with the Eevee, who are then joined by Pikachu and Axew. On the other hand, Team Rocket, drenched in water, wonder about their strange sinking experience. The trio reaches the top of the staircase, where they are attacked by the Mismagius, but Jessie calls out Woobat to counter. Elsewhere, Alexa is looking at the map and decides that it is the mountain top that the group needs to go. The three Eevee wish to come with the group too, and Axew agrees to take care of the Eevee. Suddenly, the group is attacked by Razor Leaf from a Shiftry. After missing its target, Shiftry attacks with Bullet Seed, but Ash calls out Pignite to counter with Flamethrower. After being hit by Axew's Dragon Rage, Shiftry flees but then returns with two more Shiftry. Team Rocket watches the entire commotion from a cliff and decides to sneak up on the gang. However, they end up infuriating Nidoking, who attacks them with Poison Sting, but James calls out Yamask to attack with Shadow Ball.
Ash, Iris, Cilan and Alexa successfully evade the Shiftry. Cilan deduces that there must be a reason for the Pokémon attacking them and Alexa begins to review the Pokémon they have met on this island. Upon close inspection, Alexa concludes that all the Pokémon are the same species of Pokémon that were in the pirate gang and adds that this island must be the their secret hideout. Iris then points out the Vulpix, who the pirate used to have as well, is not present on the island. Alexa states that the odds are in their favor and proposes that the group take a break. Elsewhere, Team Rocket reaches the mountain top after successfully beating Nidoking and notice the variety of statues on the mountain top. James notes that the statues look a bit odd, but Jessie shrugs him off saying all they care about is the treasure. Suddenly, the trio is attacked by a Ninetales and is left blasting off.
The group reaches the mountain top as well and notices the statues which all resemble the Pokémon they have seen before on the island. Alexa deduces that the star shape on the map must hold a secret. However, their conversation is cut short by Ninetales, who attacks the group with Flamethrower. Ash checks his Pokédex and the group concludes that the Ninetales is the pirate's Vulpix, but now evolved. After successfully evading Ninetales's Hidden Power, Ash calls out his Charizard in retaliation, who after a brief battle, ends up knocking Ninetales down in the forest. Alexa is confused, wondering that there is no treasure on the mountain top. Meanwhile, Axew, while playing with the Eevee, discovers a trench and Ash calls Oshawott to clear the dirt off the trench with Hydro Pump. Water flows through the trench in a star pattern and finally strikes the rock without a statue, and a Ninetales statue pops out of it. Cilan asks Oshawott to stop as a staircase leading from the Ninetales statue rock is discovered. However, before the group can continue with their quest, they are interrupted by Team Rocket. After reciting their motto, Jessie and James send out Woobat and Yamask respectively to battle, but are recalled back due to exhaustion from their previous battles. Pikachu then uses Thunderbolt to blast off Team Rocket again.
As the group continues down the staircase, they notice a treasure chest. Alexa opens the treasure chest, which is found to be full of evolutionary stones. Ash concludes that all the Pokémon they have seen on this island evolve by use of an evolutionary stone, with Cilan adding that evolutionary stones were much precious in the past. The three Eevee each put their paws on the Thunder Stone, Fire Stone and Water Stone, causing their evolution into Jolteon, Flareon and Vaporeon respectively. Helioptile points to the three Eeveelutions and Alexa decides to erase her recordings as a mark of respect for the Pokémon living there, with everyone agreeing to keep it a secret. Once outside, the group is greeted by all the Pokémon they met before on the island. Cilan concludes that the Pokémon were just trying to protect their home, Ash apologizes for the group's actions and Iris reassures them that they would not tell anyone about the deserted island. The group then sets off the island and continues their journey.