The episode starts at Handy's house where he is fixing one of the Piximp's peg legs, which broke. After finishing, Handy notices a seagull dropping a sea shell nearby; he picks it up and reads the writing on it. He happily exclaims, "It's an invitation from Marina!"
All of them accept, so they go to Scientist Smurf's lab who gives them a special water potion; Handy doesn't need any because he has been able to breathe water and air ever since Fooly tried to "help" him a while ago. The Piximp asks Handy, "So, this 'Atlantica' is the place where you're love, Marina, lives, right chaw?"
He answers, "Yes, and they're about to have a party. I can't wait to see my sweetheart Marina!" While hearts float around his head, Fooly interrupts with, "Marisa? But Fooly thought the Fish Girl was Job Smurf's sweetheart..." To this, Handy angrily states, "Yes, it is the Fish Girl!"
As they arrive at the River Smurf, Marina appears; after she and Handy share a sentimental moment, Joey, Acorn, Fooly, Smurfette, the Piximp, and Moxette drink the potion and dive into the water. Marina then guides them to Atlantica downstream. Once the city comes into sight, they are intrigued to find out that Atlantica is an undersea kingdom with houses made of coral. In the kingdom's center, a party was taking place and the nearer they drew, the more jubilant it sounded.
As they entered the palace, they a voice say "Marina, you've come at the right time!" Marina introduces Handy and company to her friends: Oceana, Wave, and Aqua. With brief greetings exchanged, they notice the party attractions which are similar to a carnival - Swordfish Right Toss, Dolphin Rides, and much more. They jump right into the fun.
Much later an announcement informs everyone about a dance section opening up. While some mermen play trumpets and brass-like coral instruments, the others play ocean-themed instruments (jellyfish harps, mollusk shells, etc.). Handy and Marina share their first dance while Moxette shows a bit of her sensitive side by saying, "They were made for each other" before the Piximp asks her to dance with him.
Everybody dances while Fooly fools around by playing Pin the Dorsel Fin on the Shark. His eyes are covered, and after being spun, he wanders to the wrong side of the room and leaves the party site. Wave notices him and warns Smurfette that he is going into the open sea where danger awaits. Smurfette tells her friends, and so they rush to stop him.
They swim along until they notice him approaching a resting sandbar shark; they try warn him about the danger he's entering, but when Fooly looks behind him and pulls off the blindfold, he already hits the shark with a nail. Angered for the painful awakening, it intends to eat him - luckily, Smurfette pulls him away just in time.
Then it chases them; Acorn warns Joey that Aqua is trapped in a bed of seaweed, so he uses his quick wits he obtains some pepper that Greedy gives him (having taken it before departing to the party). He tosses some in the shark's nose, making it sneeze uncontrollably. He takes the chance to get the mermaid untangled and they swim away from the shark. Aqua thanks him for saving her life by kissing his cheek to which he blushes.
They eventually return to the party and Marina and her father are relieved to learn that everyone is safe - he insists they celebrate wit more dancing, this time with a merman singing "Life in the Ocean." Aqua encourages Joey to dance with her (which was against his will at first).
They later swim back upstream to the River Smurf when the party concludes. The Smurfs, Piximp, and Pussywillow Pixie bid farewell to Marina and their three new friends. When Marina, Wave, and Oceana dive, Aqua lingers, staring at Joey and winks at him. He worries, "Oh no, another love interest!" and his friends laugh, thus ending the episode.
- While editing the smurf out of this page, Numbuh 404 had to look up shore-residing and freshwater sharks to find one best fitting the "sleeping shark" description portrayed for cartoons.