It's the morning of February 26, 1935, and the countryside near Davenry, a town in central England.

42-year-old Scottish physicist Robert Watson Watt, rocks from side to side as the van he's driving

bounces along a muddy track. Clinging to the seat beside him is his younger colleague Skip Wilkins.

With a crunch of gears, Robert turns off the road, then brings the vehicle to a skittering stop beside a field.

The cold air hits him as he steps out.

In the middle of the field is a square of four wooden poles topped with wire stretching between them.

Robert has spent the past couple of days putting them up, and he's still got blisters from digging in the hard, icy ground.

But the discomfort will be worth it if this experiment goes to plan.

A month ago, the British military came to Robert with an alarming rumor.

Intelligence reports claimed that the German army was developing a death ray,

a powerful gun that used radio waves to shoot planes out of the sky.

Robert quickly dismissed this idea as science fiction,

but it got him and his colleague Skip thinking about a different question,

whether radio waves could be used not to destroy incoming aircraft,

but to detect them.

And today, they intend to find out.

Two cables hang limply from the poles. Robert takes one and drags it to the

back of the van. Skip does the same with the second. Robert swings open the rear door. Until recently,

this was just a delivery van, but now a radio receiver has been bolted to the floor. Robert connects

the two long cables to the receiver and then switches on the machine.

Then he sits down on a stool in front of its screen and waits.

He knows that eight miles away, a Royal Air Force pilot is flying a steady circuit at 5,000 feet.

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