The Atomic Flounder is a gray irradiated flounder who is, according to the Realistic Fish Head, the result of a nuclear experiment gone horribly wrong. He became a supervillain soon after, though he has retired from villainy and is now an elderly citizen of Bikini Bottom. He wears a yellow shirt with a brown tie, a green buttoned-up sweater, and a pair of brown plaid pants. He wears a small black hat with a yellow ribbon, and uses a black umbrella with a brown handle as a cane.
In "Mermaid Man and Barnacle Boy II" and the video games, his younger self wore a blue hazmat suit with red gloves, a red belt with a golden buckle, and a radioactive symbol on the suit's chest. The suit's helmet has a green-tinted window with a red outline for the Atomic Flounder to see out of. The Flounder's face is also sideways, like a real flounder's face.
From "The Bad Guy Club for Villains" onwards, his younger self has been redesigned and now looks more like a humanoid mutant. He is now a muscular tall one-eyed fish with a shark fin, a tail, an arm growing from his right shoulder, and an eye growing from the right side of his head. He wears a tight-fitting purple shirt, orange swimming briefs, and a pair of black boots.
Due to being the result of a nuclear experiment gone wrong, the Atomic Flounder has radioactive powers. He can shoot radioactive energy from his mouth when angered, and is so radioactive that a mere touch from him can mutate a person. However, if he touches the mutated person in the same place, the mutation will be undone.
"Mermaid Man and Barnacle Boy II"
A cardboard cut-out of him appears in The Adventures of Mermaid Man and Barnacle Boy. When SpongeBob is with Mermaid Man and Barnacle Boy, they see the Atomic Flounder who is revealed to be retired. The two joke that he could turn evil again in a snap.
As they laugh, SpongeBob foolishly attacks him on the account that he is a super villain. Seeing this, Mermaid Man and Barnacle Boy tell him to stop as he explained why he attacked the old man. Enraged at this revelation, the Atomic Flounder pushes SpongeBob aside and yells at the two heroes and uses his fire breath to burn Barnacle Boy's head before angrily walking off.
"The Bad Guy Club for Villains"
In the lost episode of "The Adventures of Mermaid Man and Barnacle Boy," he was one of the villains to join the B.G.A.T.F.B.C. During the fight between the heroes and the villains, he touches Barnacle Boy and causes a hand creature to grow and control Barnacle Boy's brain.
Mermaid Man realizes that if the Atomic Flounder were to touch the hand creature, it would disappear and Barnacle Boy would wake up. After the Sinister Slug accidentally slimes the Atomic Flounder, Mermaid Man forces the Flounder to touch the hand creature, and the creature dissipates as Barnacle Boy wakes up. At the lost episode's end, the Atomic Flounder and the other villains reveal that they were actually together for a book club.
He was mentioned by Mermaid Man while asleep in the Mermalair.
The Yellow Avenger
After the Yellow Avenger defeats the Jumbo Shrimp, he speaks with Plankton and gives him some robot parts in return for information. Plankton reveals that the Atomic Flounder is back in town and is scheming in the Industrial Park. The Yellow Avenger tells Mermaid Man, who shows the sponge how to use super speed in order to fight the Atomic Flounder.
The Yellow Avenger then travels to the Industrial Park and traverses its dangers in order to find the Atomic Flounder. He eventually finds the Flounder, who notices that the hero is wearing Mermaid Man's belt and realizes that he can't fight its power. The villain decides to leave Bikini Bottom peacefully, and the next part of the game is unlocked.
SpongeBob's Truth or Square
A cardboard cut-out of him appears in the DS version during the "Mermalair" minigame, where his cut-out can be hit with Waterballs.
He appears as a cardboard cutout that the player fights in one of the nano-games.
SpongeBob Moves In!
He appears as a cardboard cutout that the player can buy and place in Bikini Bottom.
After SpongeBob, Barnacle Boy, and Patrick think that Mermaid Man has been kidnapped, they go to Atomic Flounder's home, where they find Man Ray proposing a scheme to rob a bank. While looking for Mermaid Man, SpongeBob cuts through the bank map because he thinks there is a secret pass way behind it, but there is not. The Atomic Flounder tells SpongeBob that Mermaid Man is not there, so the three continue to search elsewhere.
Later, at Jumbo Shrimp's house, the Atomic Flounder, Man Ray, and the Dirty Bubble come over. Here, they have a meal and then complain about how SpongeBob, Patrick, and Barnacle Boy have ruined their day. They all agree to get their revenge on the three. Later, they kidnap the three and tie them up to chairs at Jumbo Shrimp's house.
Soon, Mermaid Man arrives to give Barnacle Boy a present since it is his birthday, but he also gets tied up. Soon, it is time for the villains to get their revenge. Barnacle Boy has an idea. He asks SpongeBob and Patrick to sing him a birthday song while he plays the harmonica he just gets as a present. They do so and this causes the villains to get annoyed and they kick the heroes out.