In 2008, BBC cameras filmed two Swedish sisters throwing themselves into traffic on the M6. When it was shown on BBC One, nearly 7 million viewers were glued to their screens, and millions more watched it later on YouTube.
The footage was shocking. One previewer wrote "On no account miss this documentary. It opens with what is perhaps the most extraordinary footage I've seen on TV".
But this amazing footage was only part of an even more incredible story, one which could not be told at the time for legal reasons.
Now, two years later, this documentary reveals the full story of the hours just before the cameras captured that motorway footage, and the even more chilling story of what happened over next 72 hours, which left one of the sisters fleeing the scene of a crime, after she had stabbed a man through the chest.
Those who were at the centre of this fascinating legal case, including the police and Crown prosecution service, reveal the complex issues involved in both bringing charges and taking this disturbing case to trial.
A leading criminal psychiatrist, Dr. Nigel Eastman, explains the difficulties the judicial system has in achieving justice and deciding punishment when dealing with mental illness. He explains the possible causes of the womens' behaviour, and why, in his view, it could happen again.