Pedro Martín, a young and ambitious doctor, is studying the effect of cancerous cells on mice, but he has run out of mice in his laboratory, since they do not breed there, and he has no funds to purchase more of these expensive laboratory animals from the United States. Nevertheless, his assistant, Amador, informs him that he gave some of the mice to an old trapper, nicknamed “el Muecas”, who lives in precarious conditions in a shanty town outside Madrid, and that this poor man has successfully bred them with the help of the natural heat of his daughters. The incredulous scientist goes to the shanty town to obtain the mice. There, Pedro meets Muecas, Muecas's wife Ricarda and their two daughters Florita and Conchi. With the warmth of the women’s breasts, as the flirtatious Florita shows Pedro, the mice are able to reproduce.