Madrid, in the 1940s. Pedro, a young doctor, has exhausted the stock of guinea pigs necessary for his experiments in cancerology. His assistant introduces him to Muecas, the head of a large and miserable family, who secretly breeds white mice stolen from the laboratory. One night, Muecas calls Pedro to come to the aid of Florita, his daughter, who suffered from a hemorrhage following an abortion. But the doctor arrives too late to save the poor girl. Muecos fears that his incestuous paternity will be discovered. To deflect suspicion, he accuses Pedro of being responsible for Florita's death. The police immediately set off after the doctor, accused of having performed an abortion.