Heiser was a Starfleet lieutenant and a physician aboard the USS Defiant.
In 2372, Heiser was part of Julian Bashir's team when the Defiant encountered the members of the House of Gordek on the planet KDZ-E25F. Heiser was killed when remnants of a comet crashed into the location where he was treating injured Klingons. (DS9 - Day of Honor novel: Armageddon Sky)