Ides of March

What does it take to change history and destroy our reality? The same date, 15 March, six different years.
44 BC. Caesar heads to the Senate for his date with death, despite the warning about the Ides. But some have another idea for his fate.
480 BC: A massive Persian Army is invading Greece. Standing in its way: King Leonidas and 300 Spartans. More than the future of Western Civilization is at stake.
493 AD: The First King of Italy, Odoacer, is assassinated by Theodoric, But what if the sword goes the other way?
1493 AD: Christopher Columbus returns to Spain after 'discovering' the New World. But he's bringing back more than just word of his discovery; something that could wipe out mankind.
1783 AD: George Washington gives his historic speech in order to prevent a mutiny among his officers. But someone would prefer he didn't. And among his slave is one whose descendant will do write a very important book.
1917 AD: The Last Czar, Nicholas II, abdicates. He, and his family, are supposed to be taken prisoner; but the Tsarina has other plans.

The Time Patrol must send an agent back to each day, with just 24 hours for each to defeat the Shadow's plan to disrupt our time-line, creating a time tsunami and wiping our present out.