Watch " " Online HDToday. A study of the wily brown rat, humankind's unwanted companion throughout the world, whose bite on the world's food resources adds to the growing threat of shortage. In a normal, free-ranging environment, the rat is more than a match for its hosts and colonies flourish. Under abnormal conditions of restricted space and limited food, a rat colony loses all 'social' constraints on behaviour. The film has implicit analogies for all animal behaviour, including humanity's. Plagues, predators and extermination attempts are among the topics discussed.