He is a fish who found an old submarine (resembling the Nautilus) and desired to take revenge on humanity. However, even a hand-picked, fin-picked crew which includes All Hands, Mr. Starfish and some other underwater creatures didn't bring Captain Finn success.
Captain Finn is an anchovy, a small and supposedly delicious fish in some opinions. He wears a stereotypical white captain's hat and is often seen in the episode in a fish bowl when going above the waterline.
Captain Finn fulfills the role in this episode as the antagonist of the Rescue Rangers. It is worthy to note that he is a villain unlike any other the Rangers have had to suppress. He is quite simply a terrorist, a self-justified individual willing to use threats and destruction to achieve his own ends. Unlike Norton Nimnul, he seeks no profit for himself. Unlike Fat Cat, he has no want for power. Captain Finn is an intelligent individual who has chosen to fulfill his hopes of vengeance and revenge over the pursuit of all other things, and that made him a worthy foe indeed.
Captain Finn is an anchovy with revenge on his mind. He is the living (animated) embodiment of the old adage that one person's terrorist is another's freedom fighter. Having captured the Rescue Rangers, he places them in an "oxarium" and reveals his life story. Captain Finn was once set to become the Valedictorian of his graduating class of his School of Anchovies. However, before that could take place, his entire school was netted and dragged to the surface where they were most likely eaten as pizza topping and salad garnish. Enraged by this, he took to wandering the ocean floor, lost in thoughts of vengeance. One day, he came upon an abandoned submarine ("It was almost new, only 20,000 leagues on it!") and recruited a crew. Having prepared his craft, he set out on a one-anchovy terrorist campaign against his enemies, the "Land Walkers" (humanity).
Finn met with victory after victory in his personal war against the Land Walkers, namely destroying trucks carrying mackerel, "liberating" pet stores, and rampaging in restaurants, until he miscalculated Nemo the Orca. Attacking Sealand Aquarium Park, Finn sent his "strong right arms"; All Hands the Octopus to set explosive charges against the side of Nemo's tank. Although the Rangers were unable to stop the explosion, the capture of All Hands by Gadget's invention of a fishing pole 93 Times Faster than Average lead to Finn's ultimate failure.
At the behest of the Rescue Rangers, Nemo refuses Finn's offer of alliance. The Killer Whale (Orca) then proceeds to ram the Nautilus, sending Finn scrambling for cover. This action allowed Finn all the excuse he needed to add another group to his list of enemies, namely the "Air Breathers".
Capturing the Rescue Rangers when they entered the Nautilus in the Ranger Sub, Finn informs them that they will be his guests at the destruction of the county dam above their city. Despite their implores, the Rescue Rangers are unable to sway him, and he goes to listen to his favorite relaxation, namely the shipboard organ.
It was this organ which leads to Finn's downfall, for having escaped with the help of Gadget's Glass Cutter. The Rangers rig it as a pump to drain all the water from the sub in an attempt to stop the aquatic crew of the Nautilus as it approaches the dam. Although he promises Captain Finn that he would stop the Rescue Rangers, All Hands accidentally seals the doom of his Captain when he lets bubble solution (which the Rangers had been using to make diving helmets) into the organ.
Captain Finn's resolution to carry out his terrorist agenda against the Land Walkers and Air Breathers is shown when he orders Mr. Starfish to ram the dam even at the possible cost of Finn's life and the lives of his crew. But even as the water drains from inside the sub, Finn hopes for the destruction of his tormentors. He is enraged though as the sub begins to lift into the air, carried on a cushion of the bubbles from his own organ!
Bellowing in disbelief, Captain Finn and the Nautilus are then carried away, Gadget calculating his course being set towards the desert. There, by all known purposes, he, his crew, and his dreams of vengeance upon all Land Walkers and Air Breathers were destroyed with him in the boiling sun.