James Alan McHargue, 42, answered his front door in Brooksville Saturday evening to a man wearing "Groucho Marx" glasses with a false nose and moustache.
But Halloween was over; the man didn't want candy.
Armed with a silver handgun, the man shoved his way into the home and forced McHargue to the floor, say Hernando County sheriff's officials.
The intruder held the gun to McHargue's head, accusing him of being a police informant.
McHargue struggled and tried to push the gun away, then the attacker zapped him with a stun gun, sheriff's officials said.
McHargue was flown by helicopter to Tampa General Hospital, where he was in critical but stable condition Sunday afternoon, hospital officials said.
The officials declined to elaborate about his injuries.
McHargue, who has no criminal record in Hernando County, told police he saw a dark late-model Dodge van approach his home on Old Crystal River Road moments before he answered the door.