Zet was a member of the Hierarchy who kidnapped Janeway. He ordered the Doctor to steal Voyager's warp core or he could 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 imprisonned the Doctor with Janeway. HE than attempted to infilrate the Hierarchy's Surveillance Complex in order to steal information.
When Tuvok and 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 explode it. Janeway was able to stop the denotation, and Paris retrieved the core with the Delta Flyer.