It's December 24, 1923, on the ellipse in the President's Park outside the White House in Washington, D.C.

Secret Service agent Richard Jervis scans the enormous crowd that's gathered on the lawn.

His eyes flick from face to face.

Most people are wrapped up in hats, scarves, a long coat, and that makes sense it's a cold afternoon.

But Jervis knows it would also be the perfect way to conceal a weapon.

As the chief of White House security, such thinking is second nature to germs.

Tall and stern-looking, he's the one person who even President Calvin Coolidge has to obey.

Right now, Coolidge is standing with his wife and two teenage sons, looking up at the huge

fir tree that is the reason they're all here.

Nearly 50 feet high, this tree was cut

from the Green Mountains in Coolidge's home state of Vermont, and in the dimming late afternoon

light, Jervis can make out the thousands of light bulbs draped across its branches, just waiting

for President Coolidge to do the honors. Jervis checks his wristwatch. It's just before 5 p.m.

When the ceremony is scheduled to begin, and he's nervous.

This entire event is a security nightmare.

But at least Coolidge won't be making a speech afterwards.

If everything goes well, he can get the president back into the safety of the White House

in only a few minutes.

The crowd quiet, says the bell in the old post office tower a few blocks away rings out.

President Coolidge has handed an electrical switch,

and on the fifth strike of the bell, he presses the button.

In an instant, the tree erupts in light,

...