Catching History's Criminals: The Forensics Story, BBC Four, 9pm

As programmes like CSI and Waking The Dead can attest, forensic science is now an important tool in the police armoury and in the final part of surgeon Gabriel Weston's engrossing (and sometimes gruesome) history of criminal investigations, she turns her attention to one of the biggest clues detectives can have: the murder weapon. Often, the thing that does for the victim is the very thing which unmasks the killer. Simple when you know how.

Weston kicks off in China in the year 1235 and the discovery of a body in a field. It has been hacked to death with what looks like a sickle. Everyone in the village who owns one is told to lay theirs out on the ground - but there's nothing to see until flies begin to land on one of the blades, denoting the presence of microscopic traces of blood. The killer confesses and before you can say Ming Dynasty, history has its first recorded instance of forensics being used to identify a murder weapon and thus a killer. Weston, holding a mean-looking sickle, can't help grinning as she recounts the tale.

Next she tells how science developed a way of detecting arsenic - take a bow James Marsh, Scottish chemist and inventor of the Marsh Test - which until then was thought to be undetectable and therefore the poison of choice for homicidally-inclined Victorians. And with that in mind, we meet Marie Lafarge, who poisoned her husband with arsenic in 1840 and was convicted of murder in a sensational trial at which the Marsh Test was deployed as evidence.

Later Weston turns to the future of the science: developments such as three-dimensional laser scanning which can build a virtual model of a crime scene and calculate things like the trajectory of a bullet, important when different versions of events put different people in different places. She also looks at the advances in chemical analysis which can gather all sorts of information about a person's movements from a single strand of hair.

Finally, to give the programme its "Yuk" factor, she handles an actually murder weapon - the cruel-looking blade used to murder a young woman near an army barracks in Surrey in 1942 - and the skull of the victim.