The first episode of a gripping new two-part special takes viewers behind the scenes of police investigation into the death of an 11-week-old baby: a crime that leaves experienced officers shocked and dismayed by what they find but determined to pursue justice for the baby boy. Police from Cambridgeshire’s Major Crime Unit are called to a flat in St Neots after a baby has been found by his mother to be floppy and unresponsive when she returned from her school run. Detective Inspector Lucy Thomson, who leads the investigation, is an experienced murder detective but has also spent many years in child protection. As she examines the apparently well-kept home for clues, the first thing that catches her eye is that Christmas decorations are already up, on the first of November. While the baby is in intensive care watched over by the young mother and her partner, police are also intently watching the couple. Cameras follow every twist of an investigation that probes deep into the domestic arrangements of the couple, on behalf of a victim who will never be able to tell police what really happened. After 11 days the vigil ends, as tragically the baby loses his fight for life. DI Thomson orders an urgent autopsy, and the pathologist reports the scale of his injuries to be similar to those suffered by Baby P. Her case becomes a murder inquiry -and the most emotive of her career. This is real-life drama where the reality of what happened turns out to be more compelling and disturbing than fiction. Concludes tomorrow on Channel 4.