Lessing served as a Starfleet crewman 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. (VOY: "Equinox")
As a member of the Equinox crew, Crewman Lessing 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 to return to the Alpha Quadrant in mere months.
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, Lessing was assigned the task of taking internal sensors in the section containing Voyager's field generator offline, allowing them to beam it to the Equinox for use in confining the lifeforms and continuing their experiments. (VOY: "Equinox")
After successfully escaping from Voyager, the Equinox crew hid their vessel in orbit of a planet with a parthogenic atmosphere, where Lessing led a two-man landing party to localize deuterium ore deposits. While on the planet, however, the team was ambushed by a team from Voyager, and Lessing and Tassoni was taken into custody.
Later, Captain Janeway interrogated Lessing in one of Voyager's cargo bays as to Captain Ransom's tactical status, threatening to drop the shields around the bay and allow the nucleogenic lifeforms to enter. Lessing refused to cooperate, believing that Janeway was bluffing, and that she would not actually sacrifice Lessing's life to attain her goal of finding Ransom. Janeway was adamant, however, and followed through with her threat. Lessing was only saved by the intervention of Commander Chakotay, who entered the cargo bay and used a phaser to seal the interspatial fissure which the lifeforms used to gain access to the normal spacetime continuum. Realizing that Lessing would not betray his commanding officer, Chakotay instead inquired about the Ankari, the species which had introduced the Equinox crew to the nucleogenic lifeforms in the first place.
Using this information, the Voyager crew was eventually able to defeat the Equinox, and took aboard five new crewmembers, [Lessing and Tassoni included], all of whom were reduced to the rank of crewman. (VOY: "Equinox, Part II")