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To kick off the new year, we're revisiting one of our most gripping spy stories, that of Aymand Dean, the spy who betrayed Bin Laden.

A note to listeners.

This episode of The Spy Who contains depictions of violence

and may not be suitable for everyone.

December 2008, High Street Kensington Underground Station, London.

Amon Dean stands on the platform waiting for the tube train.

He's a 38-year-old Bahraini man wearing stubble, glasses and a backpack.

He looks around nervously as a security message plays over the public address system.

If you see something that doesn't look right, speak to staff or text the British Transport Police.

Dean's eyes flits side to side, checking the exits, scanning faces.

Al-Qaeda know he's betrayed them to British intelligence. Now they want him dead.

So he's shaved off his beard and changed up his travel routes to reduce the risk of being spotted.

As the train nods to a standstill, Dean ducks out of line and walks at a fast pace to a door farther up the platform,

checking in its peripheral vision to see if anyone follows.

Satisfied he's in the clear, Dean squeezes onto the train.

Excuse me.

The carriage is heaving with tourists and Christmas shoppers.

Dean moves into the well of the door and grasps one of the handrails.

Instinctively, his eyes pass up and down the rows of passengers,

a mum with two children clutching toys from the Disney store,

a businessman tapping at his Blackberry,

...