Mantine Overboard! (Japanese: マンタインとちんぼつせん！！なぞのポケモンのひみつ！ Mantine and the Sunken Ship!! The Secret Pokémon Riddle!) is the 5th episode of Pokémon: Master Quest.
After Misty caught her Corsola, Ash and co. try to get to the next ferry to Red Rock Isle, only to be too little, too late. The ferry left and another won't arrive until tomorrow. Things look bad until they meet a girl named Luka, who happens to be trying to find a Silver Wing that belonged to her great-grandfather. Will they be able to find it before Team Rocket does?
The heroes run through the town to catch a boat for Red Rock Isle. Misty tells Ash that he shouldn't have eaten a lot of meal for breakfast, else they'd make to the boat in time. However, Brock notices the boat leaving the port. As they come to the port, they see the boat is far away for them to be noticed. Upon asking a man when will the next boat come the heroes learn as it comes once a day, that was the last boat today. A woman approaches the discouraged heroes, causing Brock to be excited by her presence. The woman, Luka, studies the Pokémon that live underwater, so Brock proposes himself to be her assistant, causing Misty to pull him. Luka decides to give them help and transport them to Red Rock Isle, making Brock even more satisfied when he flirts with her.
Team Rocket spy and spot a girl with the twerps. Luka presents them a boat and even if she is aware it does not look much, she knows it will be enough to get them to Red Rock Isle. The heroes see Luka's diving gear, then spot a map. Misty thinks it is a treasure map. Luka responds she nearly guessed, as this treasure is valuable to her. The heroes see a picture of Luka's great-grandfather. Luka explains he started the research on the Water Pokémon in this sea and the job passed on to his successors. When Luka was cleaning the attic, she found a map and a journal, which states the location of a Silver Wing, which might be the key for finding a mysterious Pokémon. Ash remembers they encountered something when they were stranded at sea. At any case, Luka's great-grandfather's ship sank, leaving the Silver Wing at the bottom of the ocean. Nobody believed his story, so he went to search for it, but had no luck. Luka tells she made some research about the sea and with the map, she managed to find the location.
Ash and Brock are very excited and ask for Luka to help in finding this Silver Wing. Luka accepts their help, while Team Rocket heard this story. James wants to steal the Silver Wing, but Meowth knows the plan is to get the twerps to acquire the treasure, then they can take that and Pikachu. Jessie sees maximal profit with minimal effort is Team Rocket's way. Luka tells that they'll dive in, but that is the easy part. Later, Luka sends three Magikarp. Brock wishes Ash and Luka luck in finding the Silver Wing, while Misty warns them to be careful. Ash and Luka jump in water, while Pikachu goes to join them, but is stopped by Brock. Luka and Ash explore and soon find a ship. The Magikarp go to scout it, but get stopped by a Mantine, who guards the ship. Ash and Luka return back and try to make a strategy. Luka thinks Magikarp can distract the Mantine while they sneak into the ship. Ash thinks Pikachu's Thunderbolt will distract Mantine as well.
Soon, Luka, Ash and Pikachu (who is glad to join them) have the diving gear and swim in the water. Pikachu admires the view, while Ash is glad to have him by his side. With everything in place, the operation starts. Luka goes into the ship, while Mantine attacks Ash and Pikachu. Luka goes inside the hull and finds some two bright objects in the dark. Pikachu goes onto Mantine to attack, but gets pushed away by a Remoraid's Water Gun. Luka also gets hit by Water Gun, so she, Ash and Pikachu retreat to the ship. Team Rocket are not pleased that the twerps did not get the treasure. Jessie, however, sees that they don't need Pikachu and focuses on Mantine instead. Luka and Ash report the problem. So, Misty decides to join, since her Water Pokémon have the advantage here. Ash sends Totodile, while Misty Corsola and Staryu. Psyduck lets himself out, though Brock pulls him out of water, since he does not know how to swim.
As Misty, Luka, Ash and Pikachu dive, they notice Team Rocket's submarine being in conflict with Mantine and Remoraid. So, they sneak into the ship. After a search, Luka finds a nest of baby Remoraid, which is why the Remoraid protect the ship, but they do not know why Mantine protects the ship. The Mantine stops beating Team Rocket's submarine, so Team Rocket go into the ship, causing damage. The baby Remoraid disperse and Luka notices a chest. Luka takes the chest and goes out of the room with Ash and Misty. They notice Team Rocket's submarine, which plays a recording of Team Rocket's motto. They take the chest away and dive out. Ash, Misty and Luka follow them, only to see Mantine attacking the Submarine. James asks Jessie to drop the wish of catching Mantine, but Jessie is determined. So, Meowth releases a net to capture Mantine, but gets separated by Remoraid's Water Gun, who also push the submarine away.
Luka notices that Mantine and Remoraid live in symbiosis; they each take care of one another and protect themselves. Team Rocket release another net to capture Mantine and Remoraid, but gets cut by Totodile, Corsola and Staryu. James and Meowth convince Jessie they need to get out, so they all paddle to move out. However, Ash and Pikachu ride the Mantine and get close to the submarine. A moment later, Pikachu blasts off Team Rocket out of the sea. The chest falls down, but Ash catches the chest. Later, the heroes thank Mantine and Remoraid for helping get the chest back. Luka opens the chest and opens the box inside, revealing the Silver Wing. Luka aims to prove her grreat-grandfather's story was true and to study the mysterious Pokémon. Next day, Luka takes the heroes to Red Rock Isle, where the Whirl Cup will take place.
- This episode's name is a reference to the phrase "Man overboard!"
- This is also another episode where Misty used Ash's Pokédex.
- The "Who's that Pokémon?" is Alakazam.
Staryu's body is discolored in some scenes.