In early summer 1950, two men in clean-cut black suits step off a plane in London.

They've had a 13-hour flight from New York.

One of the men is the sycophantic right-hand man to FBI boss Jay Edgar Hoover.

And the other is one of the agency's top counter-espionage experts.

As they drive through the streets of London, they're shocked to see debris from the war

still piled high on the streets.

Sitting alongside them in the unmarked van is Jim Scarden, the MI5 interrogator who just

six months earlier had secured the confession of the most influential spy in British history.

The investigation, trial and imprisonment of Klaus Fuchs began with a fragmented decoded

message the Americans had provided the Brits.

Since then, the FBI has been continually frustrated by MI5 keeping them at arm's length.

The British establishment continually finds ways to deny the Americans' access to Fuchs

or even to his confession.

The Americans, though, are not taking this line down.

MI5 may have snared their man, but the FBI want to nail anybody who helped him when he

was on their side of the pond.

Maybe he can lift the lid on spiring still in operation.

When they arrive at Wormwood Scrubs Prison, they're taken to a grey, dank interview room.

This later, a prison warden leads in Klaus Fuchs.

The atomic physicist's face is colourless and his shoulders are stooped.

He clearly doesn't want to be here.

They all take a seat at a circular table in the middle of the room and the FBI men tentatively

...