In 2367, he served aboard the Federation freighter Arcos. During its final mission, the ship was destroyed by a warp core breach. Tan Tsu and a fellow crewmember escaped in an escape pod, and landed on the planet Turkana IV. They were captured by one of the two factions who controlled the planet, the Alliance. The Alliance demanded weapons in return for their release. When the USS Enterprise-D came to retrieve them, Tan Tsu was directed to communicate that he would be killed if their demands were not met.
Tsu and his comrade were rescued by Enterprise away team with help from Ishara Yar of the other faction, the Coalition. It turned out Yar had ulterior motives, but still allowed Tsu to be rescued. (TNG: "Legacy")