The HD-25 was found stranded on a ship in the Delta Quadrant by the USS Voyager. Used as a Maintenance Unit with extreme-hazard clearance for maintaining the ships reactor core, ejecting anti-matter waste, etc. An investigation by the Doctor and Lieutenant Torres discovered that the Isomorph had been mistreated by the crew. It was later discovered that the Isomorph had exceeded the limits of his programming but with deadly results. The Isomorph became psychotic and killed the crew of the ship. Prior to their death however they attempted to shutdown the holoprojectors in an attempt to 'kill' the Isomorph.
When Lieutenant Torres attempted to make repairs to his holographic systems, the Isomorph turned on Torres in an attempt to kill her also. The Doctor attempted to aid Torres but was knoecked offline when the Isomorph struck the portable holo-emitter causing it to shutdown. Torres was able to use a power conduit to overload the Isomorph's holo-emmitters, shutting him down.