It's early summer 1941 and Nazi planes are harassing Britain's cities.

A tall thin man enters an unremarkable red brick building under the shadow of Birmingham University's clock tower.

He makes his way down a long, grimy corridor. The windows are painted over. It's tiled in brown and thick with the smell of dust and floor wax.

At the end of the passageway, he pauses in front of double doors, bearing the discouraging sign, Tube Alloys.

Klaus Fuchs passes through and interrums where the fate of the world is being decided.

Tube Alloys is the code name for the British atomic bomb project. It's here that some of the brightest minds in the country are working in top secret to turn theory into devastating reality.

The calculations tell them a nuclear bomb is viable and Prime Minister Winston Churchill has thrown his weight behind the project.

But how to actually make it? It's a problem that Hitler scientists are likely also working on.

Whoever cracks this puzzle first will win the war.

Hello.

Fuchs's new boss, Rudolf Piles, greets him warmly.

How are you?

He is completely unaware that his new assistant is also a Soviet spy.

From the BBC World Service, I'm Rosarellis and this is season two of the bomb.

Episode three, unstable elements.

Rudolf Piles is delighted to have the brilliant young physicist Klaus Fuchs on his team.

His friendly smile and firm handshake are more than polite formalities.

He takes Klaus under his wing and welcomes him into his home.

They used to spend a lot of time with them, both working with my father and being entertained by my mother.

She was Russian. She was very loud and flamboyant and took everybody under her wing,

whether they wanted to be under her wing or not.

She would look after them.

...