A man slips into Diane's yard late at night and lurks outside the study window. Inside, Diane tells Peter that it's late and he needs to take a break. He agrees and packs away his research papers into a briefcase. Once he leaves, Cullins breaks open the window and gets the briefcase.
The next day, Peter discovers that his briefcase is missing is sure that he didn't misplace it. When he asks if Sally took it, she says that she didn't and points out the broken windowsill. Peter admits that the papers have valuable research on his condition that will take months to recreate, and calls the police. Inspector Quillan comes by and checks the yard and the window, but is unable to come up with anything useful.
That night, Peter gets a call from a Lucian Currie, who tells him that he's found Peter's briefcase in the garden and identified it from the name tag. He invites Peter over and the scientist accepts, driving himself since it's night and no one will notice his bandages. The butler, Cullins, brings Peter to Lucian's study and Lucian, a businessman, tells Peter that he had Cullins steal the briefcase so that Peter would have to come to him. When Peter tries to leave, Lucian calls in his dog Juno, an attack dog that grabs Peter by the arm. The businessman tells Peter to sit and warns him that Juno, a vicious beast that he kept alive when the local authorities ordered him to put it down, can sniff Peter out even if he removes his bandages quickly enough.
Peter has no choice but to sit, and Lucian tells him that he owns a gold mine in partnership with John Vickers. Per the terms of their partnership, the surviving partner inherits the entire mine upon the other's death. However, despite the fact that Vickers is an old man, he has clung to life for 15 years. Lucian has invited Vickers to come see him that night and wants Peter to kill him.
Vickers' bodyguard Ryan is driving his employer to Lucian's home. Vickers is grumbling that Lucian has called him out to offer him a fair price to sell his half of the mine.
Lucian explains that Vickers has a heart condition and he wants Peter to strike him a single time in the chest. He's impatient to have the mine and there's no indication that Vickers is going to die anytime soon. When Peter suggests he do it himself, Lucian explains that Vickers always keeps Ryan close at hand. He can't hit Vickers, but when Peter does it invisibly, Ryan will have to testify that Lucian didn't lay a hand on Vickers.
Peter makes a break for it and manages to get out the door and close it behind him, removing his coat and bandages. Juno goes out another door and circles around, and finally corners Peter, and Lucian tells his guest to go back to the study.
Vickers and Ryan arrive and Collins escorts them to the study, while Peter takes a seat and waits invisibly. Lucian offers a mere $2 million for the mine and Vickers dismisses it at an insult. Peter has enough and runs for the door where Ryan is standing. He gets out and props it closed with a chair, and Ryan breaks down the door. Juno attacks Ryan first, knocking him unconscious, and then goes after Peter. In the meantime, Lucian grabs Ryan's gun and advances on Vickers, and then hits his partner in the chest. Vickers collapses just as Collins comes in. They hear Juno barking and go to make sure that the dog has killed Peter, the only witness against Lucian.
Juno chases Peter into the garage and he starts up Lucian's car and smashes out through the door. Realizing that they've lost, Lucian tells Collins that they'll have to flee the country. Collins doesn't see why he has to leave since he didn't do anything, and demands his share of the money immediately in return for his silence. After a moment, Lucian agrees to the new plan now.
Peter goes to a phone box and calls to tell Diane to call the police. He then goes back to Lucian's house and sneaks around to the man's other car. Inside, Lucian gets his money from the safe but then draws a gun on Collins and tells him that he can't leave any witnesses. He takes Collins out to his other car and drives away, unaware that Peter is in the back. Collins grabs the gun but Lucian speeds up, warning that if he's shot then they'll both die in the resulting crash. However, Peter turns off the ignition and takes the key, and then grabs the gun from Collins and orders Lucian to take them back to his home.
Later, Peter calls Quillan, who brings a doctor. Vickers is alive since Ryan administered artificial respiration to him when he woke up. An irritable Vickers vows to outlive Lucian and make sure that he's put away. Peter collects his briefcase and bids Quillan good night, and goes out to his car. Juno is waiting there and Peter realizes that the dog needs a new master and has it get into the car with him.