The engineer of the freighter Arcos, who escapes its destruction in a pod only to be taken hostage by the Alliance faction beneath the failed colony of Turkana IV.
Tan Tsu is the engineer of the Federation freighter Arcos and appears in the fourth-season episode Legacy, set in 2367.
When the Arcos suffers a fatal engine failure in orbit of Turkana IV, Tan Tsu and the ship's pilot get away in an escape pod and land on the lawless colony where Tasha Yar was born. They are seized by the Alliance, one of the two gangs that rule the underground city, and held as bargaining pieces.
The rival Coalition offers to help the Enterprise recover them, sending Tasha's sister Ishara Yar as a guide. Riker's team reaches the two men and brings them out, though the rescue turns out to be cover for the Coalition's attempt to destroy the Alliance's defences.