The villain in Tom Paris' holodeck program "The Adventures of Captain Proton," which was based on early 20th-century entertainment and represented that era's view of the future. In the serialized storyline, Chaotica was a megalomaniacal alien dictator determined to conquer Earth and enslave the human race in the Mines of Mercury.
Paris first brought the "Captain Proton" program on line in early 2375. Later that same year, the program was running when the U.S.S. Voyager got stuck in an layer of subspace where photonic beings reside. Those beings visited the holodeck simulation and took it as reality, and the characters in it as actual life forms. Damage to the ship prevented the crew from shutting the holodeck down, so Doctor Chaotica continued to play out his character, ever believing he is "ruler of the cosmos," and declared war upon these photonic beings in order to conquer "The Fifth Dimension." The Voyager crew's only option to stop this war was to play characters in the story and foil Chaotica within the reality of the simulation. While Paris took on his usual role as Captain Proton, Janeway adopted the persona of Arachnia, Queen of the Spider People, and it was Chaotica's infatuation with Arachnia that ultimately led to his downfall. The character "died," but not before swearing that he would return to seek revenge.