. The music tracks that is used is Face Me during Ash's battle with Hydraxon and Maxilos, Ride(by Presence) during the final clash with the Barraki and May using the Mask of Life and Crashed on the credits.
The story begins right at the moment Kardas was defeated when May begins to complete her final task with the Mask of Life. Just as the group are about to exit the chamber, Kardas unleashes one final blast at May knocking the Mask of Life out of her hands before the dragon dies for good. Ash and the others sets chase after the Mask as once again it has a mind of its own heading for the bay. Moving too fast to be recaptured the Mask plunges into the ocean sinking fast. Misty dives after it with Starmie but the immense pressure proves too much for her and almost drowns when a stranger from the deep with the efforts of Manaphy just saves her. The stranger says a few last words saying to help the city under the ocean directly beneath Voya Nui before dying. Meanwhile at the bottom of the ocean futher than where Misty got to, the Mask lands in a seaweed field. A person called Dekar claims the Mask as he has been waiting to use the Mask to become ruler of Mahri Nui, city of the deep. As he places the Mask on the shrine, the shrine is destroyed and what looks like mutant Pokemon, Human, Reptilian Monsters called Barraki appear. Back on Voya Nui, Ash plots his next move with May to try and reclaim the Mask of Life, but starts running out of ideas remebering from Misty's attempt. Manaphy appears before them showing a secret spiral staircase leading deep underground towards the sea floor. May, suddently remembers that Manaphy and the Sea Temple were still in danger, rushes down the spiral staircase before Ash could try and explain about the depth pressure. She runs down the staircase without realising that the walls begin glowing. As May continues, far away in the claws of the Barraki named Kalmah, the Mask of Life sends out two energy zaps. One hits her giving armor and specially designed breathing equipent for the immense pressure. The other lands on the bay on Voya Nui and divides into 5 more of the same equipment in front of Ash, Misty, Brock, Dawn and one for Max's size. They soon realise that May has gone on ahead and doesn't realise the dangers ahead. May with her new armor arrives in the cold depths of Mahri Nui. She takes some time to explore the seabed before noticing Dekar under attack by the Barraki with the Mask of Life. She picks up what looks like a chain spear and hurls it at Kalmah causing him to drop the mask. May rushes forward to claim the mask but the five other Barraki Elhek, Taradox, Crabdor, Spinrua and Shardox launch small squids blinding her temperorary before Kalmah wraps his tentacle around her. Back on the surface, Ash and Dawn descend to Mahri Nui just in time to find her in Kalmah's grip. The two find what looks like an lancher. Dawn accidently hits the trigger launching red flames at Kalmah causing him to drop May and Taradox retreats with the Mask of Life followed by the other Barraki. Dekar tells them to follow him to the cavern he dwells in. After they reach the cavern, everyone introduces themselves as well as Dekar's plans for the Mask of Life. May then explains about her task with the Mask of Life which after several discussions Dekar agrees that saving the life of something is more important.
Manaphy appears before them looking weaker than ever. May tells Ash to bring Misty, Brock and Max to Mahri Nui if they can for help. After finding out they could come, May begins her quest and battle with the Barraki. After several battles and twists of fate, Ash finds the Mask of Life in a place called the pit just as Hydraxon, one of the warriors dwelling there grabs him. He is then suddently attacked by a undersea hound causing him to drop Ash and for him to see another warrior called Maxilos. The two start to battle each other, but just as Ash pulls the Mask from Hydraxon's claw, they start to attack not just each other but him as well. Ash makes a jump but falls short when the two titans grab him and as Hydraxon is about to claim the Mask of Life, May swoops in on a monstrous sea creature she tamed earlier and grabs the it and pulls Ash from the grip of the undersea warriors. After they reunite withe others, they soon discover part of the only way to save The Sea Temple would be to destroy Voya Nui. They evacuate the inhabitants and tell them to take refuge at Kanto. They destroy the cord holding Voya Nui letting it float freely. The Mask of Life starts to flash weakly meaning time was very short. May desperate to find a solution discovers from an engraving she read earlier which was to go to the core. She soon realises the hole from the destroyed cord was the entrance and Voya Nui was going to seal it. She tells Max for a promise to look after her Pokemon back in Hoenn. Max gets upset by the situation but agrees knowing it was the right thing. May quickly swims for the entrance with Voya Nui getting close. She just makes the hole as Voya Nui crashes on top sealing the way in. Back in Mahri Nui, the Barraki realising what's happened starts their revenge attack on the group more powerful than ever. Ash faced with no choice orders Buizel to use Nova Explosion which would end the battle in a draw and possibly kill him and his remaining friends. Back in the core, May starts falling through a tunnel after the seal in drained the water out. A voice tells her to save the Temple of the Sea and Manaphy would be to put on the mask and use its true power. May puts on the mask and Manaphy starts gaining power before seeing her friends about to unleash Nova Explosion. She uses its power to teleport them back to Cerulean City shore leaving the Barraki stunned in confusion. The Mask tells her that most of the people risked their lives for the same moment would be sacraficed in result, but her courage allows her to keep her spirit within the mask at the core of the world of Pokemon. May enjoys her last moments before her body is disintergrated by the Mask of Life and her spirit kept within. Back in Kanto, Ash realising May is gone, sheds a small tear for a great friend, but he senses they will meet again.
Ash: I've found the mask!
Hydraxon: Stop! I believe that mask the girl was hunting for belongs to me child.
Maxilos: That Mask is mine! Spinax, walk all over them.
Ash: Got it.
Hydraxon: You again?
Ash: Guys, a little help here?
Maxilos: Hand over that Mask.
Hydraxon: Not before I get it first Maxilos.
Hydraxon: Finally, I have... What?
Maxilos: Spinax, hunt them down.
Ash: Whoa. May?
May: We'll take it from here.