This episode is mainly Tom Paris's retelling of a mission on a water world while he is in the brig.
The episode begins with the Voyager's meeting with a new race that originates from an ocean planet. They find out that the "planet" was losing mass at an alarming rate and would be destroyed in 5 years if nothing was changed. The inhabitants believe that something at the center of the planet is responsible for its destruction; however, their best ships cannot reach that depth. Tom Paris convinces the captain to allow him to take the Delta Flyer to the center of the planet. During this conversation, he also states that he often read Jules Verne's Twenty Thousand Leagues Under the Sea when he was a child.
The Delta Flyer's journey is cut short when an undersea animal, similar to an electric eel, attacks it; however, it is determined that the reason for the planet's loss of mass is the mining of oxygen from the planet.
When Paris reports back to Captain Janeway, he is told that the planet's government will do nothing about it, and they must stop because of the Prime Directive. Paris then disobeys the captain's orders and returns back to the planet to destroy the oxygen-mining facility. Before he can, Voyager fires a depth charge at the Delta Flyer, disabling it.
Tom Paris is then reduced to the rank of Ensign and sentenced to thirty days in the brig for disobeying Captain Janeway.