Marla Gilmore was a Starfleet ensign serving aboard the USS Equinox under the command of Captain Rudolph Ransom in 2371. In that year, the Equinox was stranded in the Delta Quadrant by the alien entity known as the Caretaker.
As the Equinox's acting chief engineer, Gilmore participated in criminal experiments to augment their warp drive by murdering a series of nucleogenic lifeforms and converting their remains into a source of fuel, presenting the crew with the opportunity of returning to the Alpha Quadrant in mere months. This incurred the wrath of the lifeforms, however, who began to stage near-constant attacks on the Equinox, gradually depleting the ship's defenses. The Equinox was rescued in 2375 by the crew of the USS Voyager, another Starfleet vessel which had been stranded in the Delta Quadrant. As the Equinox crew attempted to conceal their activities from Captain Janeway, Gilmore was assigned the task of crawling through an access port and setting up transport enhancers around Voyager's field generator, allowing them to beam it to the Equinox for use in confining the lifeforms and continuing their experiments. Setting aside her claustrophobia and her own moral misgivings regarding their actions, Gilmore complied with her orders. (VOY: "Equinox")
Although the plan was successful and the Equinox crew was able to escape from Voyager, Captain Ransom eventually had a change of heart, and decided to surrender to Janeway. In response, Lieutenant Maxwell Burke, Ransom's first officer, and a small group of crewmembers mutinied, attempting to destroy Voyager. Pretending to join Burke's mutiny, Gilmore took Ransom into custody, but instead of escorting him to the brig, they went to engineering and accessed transporter control, beaming the majority of the crew, Gilmore included, to Voyager. After the deaths of Ransom and Burke, and the destruction of the Equinox, Gilmore was integrated into the Voyager crew along with four others. As a consequence for their actions against the alien lifeforms, they were stripped of rank, and their privileges were severely limited. (VOY: "Equinox, Part II")