In July 1992, 23-year-old Rachel Nickell was stabbed 49 times in front of her young son on Wimbledon Common. This fact-based drama revisits the investigation into her killing, which saw an undercover officer – known by the codename Lizzie James – used as bait to entrap the prime suspect, self-confessed lonely man Colin Stagg, who much later was proved to be innocent of the crime.