Mack is a very laid-back, black lab that rarely barks more than once.
But on this wintry day in his Iowa backyard, Mack would not shut up. His bark was frenzied, high-pitched, and incessant, his owner said.
"He just went off," said Dave Scott, of Bettendorf, Iowa, on Monday.
Annoyed, Scott said he sent his daughter to see what was wrong with the 7-year-old family pet.
The problem was not Mack - it was next-door neighbor Marcia Allen, who was lying in her backyard, slumped in the mud.
"I could just see the corner of her coat, rolling back and forth," Scott said.
He walked over to help, not realizing she was seriously ill.
"Marcia, what are you doing on the ground?'' he asked teasingly. Then she tried to speak but could only slur and one side of her face started to droop.
What had sent Mack into a canine conniption was the sight of Marcia Allen, 67, suffering a stroke.
Allen, who is now mostly fine following last month's seizure, says one minute she was fine, standing in her yard after letting her dog out, and the next she was prone, rolling in the mud.
"Mack started barking, and he never barks," she said of her neighbor's pet. "He was barking so much that I remember thinking 'stop, Mack,'' she told InsideEdition.com, and laughed.
"He was just wonderful," she said. "He was a hero. It was like this sixth sense. It was like he was saying, 'I'm not going to stop until someone comes out here.'''
Scott is proud of Mack. The pup has gained quite a following since word spread of his doggie derring-do.
"He's still got the same bad breath he's always had," his owner joked. "But now he's famous."