With the help of forensic scientists and real-life murder investigations, Horizon explores whether it is possible to commit a perfect murder. There is a test of what fictional writers call the 'perfect weapon', an icicle, which would melt away leaving no trace. And what is the most perfect murder weapon of all? The time-honoured murder method of poisoning was used in the killing of Alexander Litvinenko. But will anyone be caught, or will it become an example of a perfect murder?