Crewman Thompson was a Human Starfleet enlistee assigned to the USS Equinox as a junior science officer, and was pulled into the Delta Quadrant in 2371 by the Caretaker.
In 2376, Thompson's crew sent a distress call which was answered by the USS Voyager. When the rescue party found him, he was armed in a crew quarters, and started firing at the rescue team. It was necessary to stun him, before bringing him aboard Voyager for medical treatment. (VOY: "Equinox")
Days later when the Equinox crew stole Voyager's field generator, and Lieutenant Burke mutinied, Thompson took the tactical controls at the conn. When it was clear the ship was going to be destroyed, he suggested surrendering to Voyager, but Burke told him to follow him to the shuttlebay on Deck 3. He was killed by the nucleogenic lifeforms along with the remaining Equinox crew. (VOY: "Equinox, Part II")