A white-haired Dr. Mazi talks to his fellow club members about how he went to a haunted house on a dare, and at midnight his friends found him wandering on the road. His hair went from black to white, and he was committed for three years. Mazi has no memory of what happened. Colonel Dennis Malloy listens and accuses Mazi of cowardice, and notes that he's had men shot for the same cowardice. Mazi asks if the mercenary lacks fear, and Malloy insists that he has served in multiple armies and shot dozens of men, and insists that he's incapable of fear. Mazi offers him a wager: he owns the house, and bets $10,000 that Malloy can't spend one night alone in the house without being frightened to death. Another club member offers $5,000, and Malloy laughingly accepts.
Mazi drops Malloy off at the house and the colonel goes inside. The door closes and locks behind him on its own, and the key doesn't work. Laughter echoes through the house and blood drips on Malloy's hand from no apparent source. A flaming head appears down the hallway and Malloy draws his gun as he walks over. It appears behind him and disappears, while laughter echoes through the hallway.
Malloy finds his way to the dining room, and a table covered with rotting food. The windows are bricked over, and lightning flashes as thunder rings out. Malloy's flashlight goes out and more ghostly images come and go. The colonel lights the candles and moves away, but the ones in the dining room go out behind him. As he moves on, the glowing ghost of a soldier appears behind him. Malloy turns and shoots, and it disappears. Checking, Malloy finds blood on the floor.
The colonel finds the cellar and starts down the stairs, and the door closes and locks behind him. As he goes down, one of the stairs breaks beneath him and he falls and hits his head. As Malloy picks up his gun, more laughter echoes around him. He follows a trail of slime on the floor to a door and opens it. The soldier's ghost appears inside and advances on him, ignoring Malloy's gunfire, and then disappears. More of the slime is on the floor, along with more blood.
Malloy goes back upstairs and eats a cold dinner, and then hears someone playing the piano in the next room. He sees the soldier's ghost seated at the piano and advances on it, and it turns and raises its hands, which burst into flame. The colonel notices a hose leading up the figure's leg and steps around it, and then cuts the line. The dummy falls over and the flames go out, and Malloy chuckles in triumphant. He feels woozy but calls out to Mazi and the other club members, and accuses them of failing to terrify him.
Malloy goes to the bedroom to sleep, after cutting another hose beneath the bed. As he lies down, metal bars cross over him and a metal blade drops down and starts swinging across his neck. Malloy yells, saying that Mazi can't kill him because the bet was that he'd be scared to death. Malloy tells him that he won't be scared and goes to sleep. The next morning, the alarm clock goes off and Malloy wakes up. The bars and the blade are gone.
Malloy goes to the kitchen and a camera lowers itself from the ceiling. Malloy asks why Mazi, who has removed his white wig, went to such elaborate lengths, and Mazi invites him to have breakfast on the automated stove. He then explains that he drugged Malloy's coffee thermos the night before, and says that Malloy has his breaking point. Mazi talks about all of the attacks that Malloy lead, including one where he poured gasoline on Mazi's father, a concert pianist serving in the Italian Army, and set his hands on fire. Malloy remembers the incident, and Mazi explains that he swore to his father that he would break Malloy just as Malloy broke his father. Mazi then tells Malloy that he's a biochemist, and has created a way to break down an enzyme in the human body, turning a man into an annelid... or earthworm. He says that he performed the process on his colleague, and gave an injection to Malloy while he was sleeping off the drugged coffee. Malloy thinks that he's bluffing, but Mazi tells him to go to the cellar and look at his colleague. When Malloy hesitates, Mazi tells him to see his fate.
Malloy takes his gun and goes to the cellar door, and sees a trail of slime leading from it. The colonel backs away from the door, goes to the camera, and tells Mazi that he loses. He takes his gun and kills himself, and Mazi says that he lost: there was nothing in the cellar.