Doctor Chaotica was a holodeck character in The Adventures of Captain Proton. As the self-styled "ruler of the cosmos", Chaotica was the sworn enemy of the planet Earth, and was constantly trying to conquer the world, with the aid of an army of minions, led by his faithful, if unintelligent henchman Lonzak. Chaotica's nemesis was none other than Captain Proton, the "scourge of intergalactic evil." (VOY: "Night")
In 2375, the program was running when USS Voyager was stuck in subspace, where photonic lifeforms who lived there mistook the program as being a true representation of the universe Voyager resided in. The aliens visited Dr. Chaotica's lair believing that they were meeting a real person. Dr. Chaotica told them that he would crush them like insects and promptly executed one of the aliens. The other transported away. This triggered a war between the aliens and Chaotica. The program could not be shut down due to damage to Voyager, so in order to end the war with the aliens, the crew assumed various roles in the program. As usual, Tom Paris became Captain Proton himself, with Harry Kim as his sidekick, Buster Kincaid. The Doctor took on the persona of the President of Earth, in order to communicate with the photonic aliens, who dismissed the carbon-based Voyager crew members as "illusions".
Most importantly, Captain Kathryn Janeway assumed the role of Queen Arachnia, in whom Dr. Chaotica had a romantic interest. Dr. Chaotica made Arachnia promise to marry him if he would drop his shields. He later imprisoned her and told her that he would not kill her until after the wedding night. Janeway and the rest of the crew were able to defeat Chaotica, stopping the war with the aliens and releasing Voyager from subspace. As Dr. Chaotica was dying, he told Arachnia that he would return. (VOY: "Bride of Chaotica!")
In 2377, when Voyager was separated into several different timeframes, one of the frames on the holodeck played host to Captain Proton again. When Chakotay and a past Captain Janeway were attempting to save the ship, they were caught on the malfunctioning holoprogram. Chaotica believed Janeway was Arachnia and vowed to kill her. However, the two "convince" him of a "greater threat", aliens from the Eighth Dimension (Chaotica and Lonzak laughed at first, because "everyone" knew there were only five dimensions) and they showed a piece of Voyager's gel-packs as "proof". (VOY: "Shattered")