The man tasked with finding and killing alleged IRA informers was an informer all along. Reporter Mark Horgan traces the story of the secret British Army Agent known as Stakeknife.

Alex trawls through Lord Lucan’s belongings from his speedily abandoned flat.

She finds incriminating books where he’s torn out pages on how to kill your wife, and is taken aback by photos that make her reconsider the story.

She draws together what makes this a compelling crime, and asks what would give it the perfect ending.

And in a remarkable interview with a former Met Police Detective, she discovers that we could perhaps get an answer to one of the two mysteries tomorrow.

Presenter: Alex von Tunzelmann

Our interest in Lord Lucan could have petered out after the inquest. But people start to spot him all over the world. Could he really have escaped the UK?

Alex von Tunzelmann explores what role this idea plays in our fixation with the Lucan case.

She hears how the media kept the story going, inventing sightings for copy and jollies abroad. People admit to elaborate hoaxes and blatantly fabricating stories.

But should we dismiss the idea? Alex finds one story from a closed police file that completely bowls her over.

Producer: Sarah Bowen

The nation was spellbound by the inquest into Sandra Rivett’s death.

For the press the story was a dream. A tale of the aristocracy, gambling, debt and murder was a welcome relief in an era of shortages and strikes. They salivated over the grim details.

Alex von Tunzelmann hears how inquest became a trial, supercharging our obsession with this case.

And she wonders if we can take his guilt as fact when she hears a never before broadcast recording of an interview of Lady Lucan and an incriminating new story from a policeman.

Producer: Sarah Bowen

Police traipsed through 46 Lower Belgrave St on the night of Sandra Rivett’s murder, but did they contaminate the evidence?

The police files are still closed. Where there have been unanswered questions, enticing myths and conspiracies have filled the void.

Alex von Tunzelmann pieces together what we can know of the investigation, trying to separate fact from fiction.

She hears from two policemen who worked on the Lucan case and reassesses the forensics with an ex-Metropolitan Police detective.

Stories emerge about close relations between the press and police and she wonders if booze, bribes and class deference may have obscured the truth.

Producer: Sarah Bowen

With Sandra Rivett lying dead in the basement, Lucan must decide whether to face the police or run.

And so begins the second mystery that has made this case so compelling.

Where did Lucan go that night?

Was he being sheltered by his friends who the police nicknamed The Eaton Square Mafia?

Alex von Tunzelmann pieces together what we know of the hours after the murder, asking whose version should we believe.

She meets an eyewitness who says she was the last person to see Lucan alive, and crawls underground into a bunker where the police were sure he was hiding.

Producer: Sarah Bowen

Many relationships have tricky patches, with couples struggling for money or over the children. But there’s a spark in this story that takes us from the Lucan’s glamorous society wedding to Sandra Rivett being murdered, Lady Lucan attacked and the children swept away. And it grips us.

Alex von Tunzleman hunts for what triggers this story, delving through a box of Lucan’s possessions not seen for decades. As she discovers cheque stubs, invoices and letters from the bank, she sees the reality of what life was like behind the Lucan’s veneer of respectability: a world of debt and alcohol, gaslighting, late night calls and stalking.

Producer: Sarah Bowen

The press portrayed Lady Lucan as a perfect victim: a tiny, fragile woman, attacked by a cruel and vengeful man. Lord Lucan’s friends said she was a volatile, difficult snob.

She is certainly enigmatic and intriguing.

With access to never before broadcast tapes, Alex von Tunzlemann looks for the truth behind the stories of the Countess and explores how her image emerged.

Lord Lucan raced powerboats through the English Channel, drove an Aston Martin around the Grand Canyon and completed the Cresta Run at Saint-Moritz.

Throwing himself into gambling, he was nicknamed Lucky.

With slicked back hair, charm, style and humour, he was remarkably striking.

But underneath this image there was a very different man.

Alex hears from friends, biographers and someone who lived in the Lucan household to ask who really was Lord Lucan?

Was he the vicious, violent and dangerous murderer of Sandra Rivett or was this a myth it was seductive to believe?

Producer: Sarah Bowen

What really happened on the night of 7th November 1974 when Sandra Rivett was murdered?

Young reporter Bob Strange sneaked into the hallway at the Lucan’s house in Belgravia. The police burst in trampling through the crime scene.

The evidence of the Lucan case is murky. There are many versions that contradict themselves. Lord Lucan says something entirely different happened.

Who should we believe? And how do the mysteries surrounding that night, drive our obsession with this case?

Producer: Sarah Bowen