The sound of the cuckoo is to many the very essence of spring, yet behind the magical call is a bird that is a cheat, a thief and a killer. Just how does the cuckoo trick other birds into accepting its eggs and raising its young? Why don't the duped foster parents react as they watch the baby cuckoo destroy their own eggs and chicks? And why do they work so relentlessly to feed a demanding chick that looks nothing like them and will soon dwarf them? In this film, new photography is combined with archive footage and the latest scientific findings to solve a puzzle which, as narrator David Attenborough explains, has perplexed nature watchers for thousands of years.