A young Spanish couple, medical students in Boston, return to Spain years after graduating. With a fine academic background, their children and their dog, they intend to carry out promising medical research: the early detection of cancer through the canine sense of smell. Preliminary results show the effectiveness of the hyper-developed canine sense of smell in detecting tumour cells. An accident cuts short the researchers' future, and only their daughter Sofia and their faithful pet, Romel, survive. The girl is left in the care of her maternal grandparents in Madrid and a shepherd takes care of Rómel, but he escapes and sets off from Asturias to Madrid, following Sofía's trail, to try to reunite with her and help her with her depression.