In the Yukon Territory in 1931, solitary American trapper Albert Johnson (Charles Bronson), comes across an organized dog fight. One of the dogs, a white German Shepherd, is badly injured and Johnson forcibly takes it, paying $200 to its owner, a vicious trapper named Hazel (Ed Lauter).
Aggrieved by his treatment and claiming the dog was stolen from him, Hazel, leads several of his thugs to Johnson's isolated cabin. The men then attack the cabin, shooting while others create a diversion. Johnson kills one of the men, a trapper named Jimmy Tom (Denis Lacroix), after he shoots Sitka, the dog, which Johnson has nursed back to health.
Once they discover that Johnson has bought 700 rounds of ammunition from the local trading post and paid in $100 bills, the trappers become convinced that Johnson is the "mad trapper", a possibly mythical, psychopathic, serial-killing figure who supposedly murders other trappers in the wilderness and takes their gold teeth.
An old trapper, named Bill Lusk (Henry Beckman), warns Johnson that the law is coming for him. Johnson fortifies his cabin.
Sergeant Edgar Millen (Lee Marvin), commander of the local Royal Canadian Mounted Police post in this region, seems a tough but humane man. He has a tracker named Sundog Brown (Carl Weathers), and a new constable named Alvin Adams (Andrew Stevens), plus a new lover in fellow mountie Vanessa McBride (Angie Dickinson).
Millen leads a posse of armed mounties and trappers to the cabin. He meets with Johnson, who comes out his front door, telling him that he has a pretty good idea of what happened and if Johnson comes with him they can get it sorted out. However, before Johnson can answer, one of the trigger-happy trappers opens fire and Johnson is forced to run back inside and engages the trappers in a shootout. Several of the trappers end up killed, including one who is shot for screaming by one of his friends. The posse uses dynamite to blow up the cabin, but Johnson escapes, shooting dead a mountie, Constable Hawkins (Jon Cedar).
Millen, Sundog and Adams, joined by Hazel with his tracker dogs, set off into the frozen wilderness after Johnson. The case has made front-page news across the country, so many trappers also set off after the RCMP, attracted by a $1,000 bounty that has been placed on Johnson's life. Captain Hank Tucker (Scott Hylands), a Royal Canadian Air Force pilot, is also sent by the government to join the hunt, which is causing a national embarrassment. He reveals that Johnson was a member of a United States Army special intelligence unit during World War I.
Johnson utilizes a number of tracking techniques to avoid Millen's posse and the bounty hunters, living off the land in treacherous winter conditions. As the hunt continues, Millen begins to respect Johnson's uncommon abilities while growing to resent the intrusion of so many outsiders.
Lusk comes across two of the trappers camping in the wilderness and shoots them both dead before pulling out their gold teeth. Lusk, so it would appear, is the real mad trapper.
The pursuers catch up to Johnson. Tucker, flying his biplane, begins to carelessly machine-gun the area, killing Sundog. The enraged Millen and Adams shoot down the plane with their rifles; Tucker crashes into a canyon wall and is killed. Johnson escapes after killing Hazel.
Lusk comes across Johnson and tries to kill him, presumably attracted by the reward. Johnson tricks him and captures him at gunpoint. Millen spots Johnson and opens fire; the bullet hits him in the face, rendering him unrecognisable. As they examine the body, both Millen and Adams spot the real Johnson, dressed in Lusk's clothes, on a ridge above them. The man they shot was Lusk dressed in Johnson's clothes.
The mounties allow Johnson to flee into Alaska, well aware that everything he did was in self-defense. As the other pursuers appear, Adams tells them that Millen has killed Johnson. A trapper finds that the body has a pocket full of gold teeth, so they celebrate the killing of the "mad trapper."