At a manor in the tropics of Borneo, Stephen Macy returns home out of the rain and complains about the weather to his employer John and his much younger wife Rhona Warwick. They are satisfied with their lot, and John is clearly happy in his marriage. Once he leaves, promising to return in a quarter hour, Macy approaches Rhona and comments on how easy she is to please. He compliments her on her looks and asks how she can stand to be in Borneo with a husband old enough to be her father. Rhona tells him to stop and warns him never to bring up her husband's age again. She accepts his apology and tells him to take each day as it comes, but Macy says that he wouldn't force a wife to stay in such a tropical hell. However, he assures her that he signed a contract to serve there for a year and will honor it, and makes her say that they're still friends. The kindling salesman, Tommy Robinson, comes in and sees them together, and Rhona dismisses his implication they were doing anything untoward.
Once Rhona leaves, Tommy introduces himself and asks how Macy finds the weather. He explains that he was sent there to escape a prison sentence, and invites Macy to come to the village sometime to share a drink. Tommy then suggests he might offer Rhona to him. When Macy doesn't know what he means, Tommy promises that they'll talk about it later. He goes out past John, who warns Macy that Tommy is a scoundrel. As Rhona comes out, Macy admits that he has an appetite, and it's growing. Once John leaves, Rhona smiling tells Macy that she knows what abstinence can do for a man. When he asks her to continue, she tells him to take a cold bath and walks away.
Later, Macy goes to the village tavern and arranges a meeting with Tommy. Tommy assures him that it's normal to seek out others, and that he can understand that Macy wants to be closer to Rhona. Macy claims not to know what to do, but Tommy says that they both know what he wants. When Macy says that he doesn't want an assassin, Tommy insists that he is no killer and offers an act of destiny. He explains that Borneo is home to the earwig, a caterpillar that feeds on wax and enjoys the human ear. Once in the ear, it can't back out so it continues to feed as it goes straight through. Macy insists that he wants no part of it and starts to leave, but Tommy stops him and explains that he has some friends who can sneak in and do what is necessary. It will then take two to three weeks for the victim to die, weeks of excruciating pain. Macy demands the cost, and Tommy tells him it will cost a hundred pounds. After a few minutes of consideration, Macy starts to take out his money. He tells Tommy that he doesn't want to see him again, but the criminal says that the only difference between him and Macy is that Macy doesn't want blood on his hands, and they'll meet again.
The next day, Macy enjoys breakfast with the Warwicks and begins to feel a pain in his ear. He checks it and finds blood, and realizes that the earwig is in his ear. Ranting that they put it in his ear, Macy runs off in horror.
Two weeks later, the doctor is forced to tie the agonized Macy to his bed to prevent him from tearing at his own ears. As he leaves, Tommy comes running up but the doctor says there's nothing anyone can do. Tommy goes to Macy's window to apologize for the man he hired coming to the wrong room. Macy says that he wants to die from the pain, and Tommy assures him that he will. He apologizes profusely and assures Macy that it won't last much longer.
Later, the doctor tells John and Rhona that in defiance of all odds, the earwig went all the way through Macy's brain without damaging it, and came out on the other side. Macy comes in and the doctor congratulates him on benefiting from a miracle. Rhona, well aware of what he intended, says that she wouldn't have turned to him once John was dead. Macy says he would have murdered any number of men for love, and he paid the price with two weeks of suffering. He asks John if he'll be arrested, and John says that he won't. Macy figures that his romantic rival doesn't want the scandal and assumes that he is free to leave. He assures the Warwicks that he'll be on the next boat, and describes what he went through to the doctor.
John insists that he can go, and Macy realizes that there's something they haven't told him. He assumes that John plans to have him arrested when he arrives in England, but they say nothing. Finally, John tells him they have no intention of having him arrested or brought to trial. He says that execution would be a mercy compared to what he has to face, and the doctor tells Macy to sit down. Once Macy does, the doctor explains that he examined the earwig when it came out, and discovered that it was female... and the female earwig lays eggs. Macy clutches at his ear and screams in horror as he realizes his ultimate fate.