Mrs. Bunting opens an apartment building with her husband, and at first things go very badly. But after some time, the Buntings have a new tenant: Mr. Sloot, a polite and well-mannered young man, ready to pay generously for rooms. True, he has his own oddities: he is religious to the point of fanaticism, constantly disappears somewhere at night, and then conducts some mysterious experiments. Meanwhile, rumors about a serial killer spread throughout London. A brutal maniac kills young women and leaves tiny pieces of paper on the victims' bodies with the words "Avenger" written on them. The police patrol the streets day and night, but they can’t get on his trail. Gradually, Mrs. Bunting begins to suspect that their tenant may not be who he claims to be...