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June 1945, Los Alamos, New Mexico.

In an office on a secret military base in the desert,

Klaus Fuchs places a sheaf of documents into his briefcase.

He's a thin 33-year-old nuclear physicist with a high hairline and wire-rimmed spectacles.

As he secures the clasps, he attracts the attention of a colleague.

Which girl are you meeting this time, Fuchs?

But Fuchs is too wrapped in his thoughts to notice his colleague's wisecrack.

Fux is a German physicist who came to Britain to escape the Nazis. Now he's in America working on the Manhattan Project,

the top secret program to build the first atomic bomb.

The hope is this devastating weapon will end the Second World War. Fuchs picks up his

briefcase and heads for the door. Klaus, is everything okay? Oh, sorry, I was lost in maths.

We've been through this, Fuchs. It's math. Mathematics then. I've got a leave day to use up before the end of the

month. Thought I'd visit the museum in Santa Fe. Pull my head out of the numbers for a bit. I'll be back for

Oppenheimer's briefing. Fuchs heads outside and clambers into his buoys.

He drives slowly through the Los Alamos complex.

Dirt roads weave through a patchwork of hastily constructed wooden barracks and functional buildings.

In the distance, the Chames Mountains rise against a clear sky.

A narrow unpaved street cuts through the makeshift town.

Military vehicles kick up clouds of dust as they rumble past. Residents walk briskly,

soldiers idle at strategic points. Fuchs drives towards the checkpoint at the entrance to the base.

As he approaches, an armed guard steps from out of his hut and waves at him to stop.

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