Mystery on a Deserted Island! (Japanese: お宝の謎！無人島アドベンチャー！！ The Mystery of the Treasure! Adventure on an Uninhabited Island!!) is the 38th episode of Pokémon: BW Adventures in Unova and Beyond.
Ash and co. are joined by Alexa, an expert reporter from the Kalos region, and her Pokémon Helioptile and Gogoat to search for a Noivern. They stop by an island rumored to have a secret treasure and various Pokémon. What mysterious treasures will they find there? And is there a theme to the Pokémon on this island?
Alexa sails on a ship with Helioptile to Frond Island. The heroes greet Alexa, who shows them the island. She also shows a coded message for a treasure map, shocking the heroes. Alexa admits she visited a pirate museum a while back and heard about a pirate captain, who had many Pokémon with him and sailed across the sea. He supposedly left a treasure on the island, but nobody knows what it actually is. Alexa, however, managed to find a copy of the map and the code leading to the treasure, for it is supposed to lie on an island near Frond Island. The heroes decide to join in the treasure hunt. Team Rocket overhears this and decides to follow the twerps and snatch the treasure for themselves. They think it might be money, while Meowth thinks it may be gold, silver, rubies, sapphires, emeralds, diamonds or pearls.
Taking a boat, Alexa and the heroes sail for the treasure island. Alexa is certain there is a passage to the island. They find it, but are swept by a whirlpool. After a while, the heroes and Alexa wake up, seeing the whirlpool was an obstacle. They go through the passage and encounter three Honchkrow, who are staring at them. The heroes pass by, while Team Rocket is in their Basculin submarine. The heroes sail with Alexa and find an abandoned ship. Pikachu knocks on the hull, causing a hole to be opened. Cilan doubts it is a good idea, but Alexa and Iris think they must not let be scared so easily. Team Rocket paddles faster to catch up, but tackle the ship, causing it to shake. The heroes immediately go out and see the ship going back into the sea, sinking down.
However, Pikachu notices a glyph door. Alexa sees her codes should decipher the glyphs, but they do not match each other. Ash pushes a stone, causing it to slide down. Cilan realizes it is a puzzle door, so they need to align the glyphs into a correct order. A Honchkrow observes them and flies off. After a while, the heroes managed to solve the puzzle, opening the door and revealing a staircase. The heroes charge in with Alexa and encounter Mismagius, who fire Hidden Power and Shadow Ball at them. Ash and Iris send Snivy and Dragonite, who dodge Hidden Power. Using Leaf Blade and Ice Beam, Snivy and Dragonite hit the Mismagius. The heroes go further, but encounter a Nidoking scaring some Eevee. Pikachu goes to use Iron Tail, but Nidoking walks away.
Cilan knows the Eevee are just hungry, so gives them some of his Pokémon food, with Axew giving them some berries. The Eevee are friendly and start playing with Pikachu, Axew and Helioptile. Team Rocket dives out of the sea, since they hit the ship with their submarine. They run to catch up to the twerps, but are attacked by Mismagius' Shadow Ball. Jessie sends Woobat, who uses Air Slash against their Hidden Power. Alexa notices they need to go at the top with a certain symbol. The heroes continue on, accompanied with the Eevee. Suddenly, they are attacked by Shiftry's Razor Leaf, followed with Bullet Seed. Ash sends Pignite, who counters the Bullet Seed with Flamethrower. Axew uses Dragon Rage, hitting Shiftry, who flees and returns with two more Shiftry. Team Rocket sees the twerps distracted, so jump down to get ahead of them and get the treasure first. However, they land on Nidoking and flee from his Poison Sting. James sends Yamask, who hits Nidoking with Shadow Ball.
The heroes stop to catch breath. Alexa reviews the Pokémon they encountered and realizes they belong to the captain, aside Vulpix, realizing this was the captain's secret hideout. While the heroes rest, Team Rocket came to the top of the mountain, but see only statues and no treasures. However, they encounter a Ninetales, who blasts them off with Hidden Power. The heroes also arrive to the top and encounter the statues. As Alexa takes footage, she warns them of Ninetales. They realize it is the captain's Vulpix, who evolved. Ninetales attacks with Energy Ball, so Ash sends Charizard, who uses Flamethrower, but misses. Ninetales uses Energy Ball, hitting Charizard. Charizard uses Dragon Tail, but misses, due to Ninetales' Double Team. Charizard dodges Ninetales' Flamethrower and hits it back with Slash. Using Dragon Tail, Charizard hits Ninetales, sending it into the forest.
However, the heroes see no treasure. Axew removes some dirt, seeing it covers a trench. Ash sends Oshawott, who uses Hydro Pump to remove the dirt. This pushes the statues, revealing the fifth one of a Ninetales, as well as revealing an entrance to a cave. Surprisingly, Team Rocket returns to stop the twerps. Jessie sends Woobat and James sends Yamask. However, Yamask and Woobat are exhausted from the eariler battles, so Pikachu blasts Team Rocket off easily with a Thunderbolt. The heroes and Alexa enter the cave and encounter the treasure chest. Alexa opens the treasure chest, revealing the nine evolutionary stones - Fire, Thunder, Water, Leaf, Moon, Sun, Shiny, Dusk and Dawn Stone. The heroes realize the Pokémon on this island are the ones that evolve using the Evolutionary Stones. The Eevee touch the stones, evolving themselves into a Flareon, Jolteon and Vaporeon.
Helioptile warns Alexa, who understands and erases everything she recorded on the island. She fears if the public knew about the island, they would get their hands on the evolutionary stones, so decides to keep this a secret. The heroes exit the cave and encounter the Pokémon they battled. They promise not to tell anyone about this secret and take a boat back to the cruiser.
In the dub, Meowth mentions Gold, Silver, Rubies, Sapphires, Emeralds, Diamonds and Pearls. This is reference to the Generation II, III and IV games.
In Cilan's demo, a Nidoran♂ evolved into a Nidoking with a Moon Stone without going through the Nidorino stage.