Dax was a Zeosian who served in Starfleet during the 23rd century. As of 2293 he held the rank of crewman and served aboard the USS Enterprise. During his time aboard the ship, evidence pointed to Dax being one of the assassins to kill Chancellor Gorkon. The incriminating evidence was a pair of gravity boots. Pavel Chekov used them to accuse the Zeosian before it was brought to Chekov's attention that Zeosian feet would not fit into the boots. Lieutenant Valeris ended up being responsible for framing Dax. (TOS movie: The Undiscovered Country, ST website: Startrek.com)