Hagler was one of several officers abducted by the solanogen-based lifeforms and subjected to bizarre medical experimentation. One such procedure caused his blood to transform into a liquid polymer. The alterations were irreversible, and Hagler died shortly after being returned to the Enterprise-D. (TNG: "Schisms")
Hagler was played by actor Tyce Bune, who received no credit for this appearance.
Although Worf states Hagler's quarters to be on Deck 9 Section 17, Dr. Crusher later calls for a plasma infusion unit to Deck 9 Section 19, even when she is directly outside his quarters.