Zet was a member of the Hierarchy who kidnapped Captain Janeway. He ordered The Doctor to steal USS Voyager's warp core or he would kill Janeway. The Doctor, after impersonating various crew members on Voyager, returned to Zet's ship with the core. However, Zet refused to live up to the deal, and imprisoned The Doctor with Janeway. He then attempted to infiltrate the Hierarchy Central Command in order to steal information by using The Doctor as his agent.
When Tuvok and Tom Paris tracked down Zet's ship in a shuttle from Voyager, they engaged in a phaser fight with him. Zet jettisoned Voyager's warp core and was going to destroy it. Janeway was able to stop the denotation, and Paris retrieved the core with the Delta Flyer. (VOY: "Renaissance Man")