Angelo Tassoni was a crewmember 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 a member of the Equinox crew, Tassoni 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. 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 2376 by the crew of the USS Voyager, another Starfleet vessel which had been stranded in the Delta Quadrant. The Equinox crew attempted to conceal their activities from Captain Janeway, and eventually stole Voyager's field generator for use in confining the lifeforms and continuing their experiments. (VOY: "Equinox")
The Equinox crew hid their vessel in orbit of a planet with a parthogenic atmosphere, where Noah Lessing led a landing party of himself and Tassoni to localize deuterium ore deposits. While on the planet, however, the team was ambushed by a team from Voyager, and both were taken into custody.
After the death of Captain Ransom and the destruction of the Equinox, Tassoni was integrated into Voyager's crew along with four other Equinox crewmembers. As a consequence for their actions against the alien lifeforms, they were stripped of rank, their privileges were severely limited, and they were expected to work under extremely close supervision. (VOY: "Equinox", "Equinox, Part II")