Our heroes take an excursion to the Muraille Coast before Ash’s Cyllage City Gym battle. They spend some time fishing until Ash pulls up a Skrelp, which attacks Pikachu with its poison! Fortunately, they meet a couple along the beach road who heal Pikachu with a handy Antidote. Eddy and Lindsey are underwater archeologists in search of the Cussler, a luxury liner that sank many years ago. Our heroes decide to help them with their quest!

While Clemont and Bonnie stay above with Eddy, Ash and Serena join Lindsey in her submarine, and they dive toward the ocean floor. Sure enough, the wreck of the Cussler is there, and thanks to some crafty Dragalge who have fused it together with Acid, it’s become a home for many different kinds of Water-type Pokémon!

A mysterious light lures them deeper into the wreckage, where they spot Team Rocket’s submarine. The villains have come to claim the Cussler’s sunken treasure, and their meddling has endangered the structure of the ship. With a little help from our heroes, the wild Pokémon manage to fix up their underwater refuge. But at the same time, Team Rocket escapes with the Cussler’s safe—only to get caught in a whirlpool! Skrelp is caught too, but Froakie comes to the rescue, showing off its spectacular jumping skills…and giving Ash some ideas he would put to use in his upcoming Gym battle against Grant!