In Paris, hotel owner Marguerite is having her grandson Douglas from America stay with her for the summer. One of the other boarders is Mr. Dumas, an artist who paints on the banks of the Seine. Douglas talks to him briefly and then comes inside to watch Marguerite gutting a chicken with a large knife, stuffing it, and sewing it back together. The boy goes to the window and sees a new boarder approaching, a man using an umbrella despite the fact that it's a sunny day with no rain.
The applicant rings the doorbell and introduces himself as Mr. Koberman. He wants to rent a room from Marguerite and she agrees despite Douglas' efforts to stop her. She has Douglas take Koberman up to his room. The boy opens the drapes and notices with interest as Koberman winces in pain at the sunlight streaming into the room.
Later, Douglas sneaks out onto the balcony and spies on Koberman, who is sleeping on his bed in a deathlike trance. That night, Marguerite has everyone come down to supper. As they prepare to eat, Mr. Dumas mentions that a number of young girl shave gone missing recently. He warns Emily, a student, that she should be careful. Koberman comes in and says that he's a traveler with no fixed employment. When Marguerite sets out the silverware, Koberman takes out his own eating utensils made out of wood and apologizes, saying that the sound of silver unnerves him. He tells Marguerite not to prepare him regular meals as he comes and goes throughout the day. Douglas strokes a silver fork to entertain Emmy, amplifying the noise with a crystal goblet, and Koberman abruptly leaves in discomfort.
That night, Douglas is showing off his collection of silver dimes to Dumas when Emmy leaves to go to the library to study. Douglas assures her that he'll protect her if he has to. A few seconds later, Koberman prepares to go out and Douglas watches him leave. Outside, Koberman secretly follows Emmy into the night.
The next morning, Douglas is in his room playing with a toy gun with a filtered viewing scope. He sees Koberman walking to the hotel and looks at him through the scope. Several strange organs glow inside Koberman, revealed by the filtered scope. Koberman spots Douglas and comes upstairs to talk to the boy. As they talk, Koberman toys with the gun and then drops it out the window. He claims it was an accident and promises to buy Douglas a new one. As he goes, Douglas notices that the new boarder has very long fingernails. Once Koberman is gone, Douglas goes downstairs and salvages the lenses from the gun scope.
At breakfast, everyone notices that Emmy isn't there. Douglas goes to her room and discovers she didn't sleep there the previous night. He checks on Koberman and discovers him sleeping again in the same deathlike trance. Using the lenses, he finds the same strange organs in Koberman's chest. As he looks around the room, he sees dozens of old photos, all of them featuring young women. Douglas finds a picture of Koberman standing in front of the Eiffel Tower while it's under construction in 1869. The last thing the boy discovers is Koberman's newest photo: Emmy.
Douglas borrows Dumas' books on human anatomy and asks him about vampires. The boy wonders if vampires really exist, but figures that if they do they may be nothing like the traditional vampires. "Real" vampires might be able to walk in daylight, and hide themselves by acting like normal humans. He wonders if vampires are afraid of silver like the legends say.
That night, Douglas gets his grandmother's knife and his collection of dimes, and goes to Koberman's room. The man is in a trance again. Douglas raises the knife high and plunges it into Koberman's chest. Once he's done with his grisly task, Douglas takes Koberman's heart-organ and shows it to his grandmother... still beating.
Marguerite calls the police and they examine Koberman's body. The coroner confirms that Douglas cut the organ out of Koberman's chest, and that the "man" wasn't human. Douglas has confirmed that Koberman was still living even after his organs were removed, and the police wonder how Douglas killed the creature. The coroner cuts open the chest wound, which Douglas has carefully stitched back together, and discovers that the boy poured his collection of silver dimes into the vampire's chest to destroy it once and for all.