There once was a place called Atlantis. 10,000 years ago, it was attacked by a force known only as the Shadow, on the same day over the course of six years. The last attack led to Atlantis being obliterated to the point where it is just a legend.
There are many Earth timelines. The Shadow comes from one of those alternate timelines (or perhaps more than one). It is attacking our timeline by punching bubbles into our past that can last no more than 24 hours. In each bubble, the Shadow is trying to change our history and cause a time ripple.
By itself, a single time ripple can be dealt with, corrected, absorbed. But a significant time ripple that is unchecked can become a Cascade. And six Cascades combine to become a Time Tsunami.
That would be the end of our timeline. And our existence.
To achieve its goal, the Shadow attacks six points in time simultaneously; the same date, in different years.
The Time Patrol’s job to keep our timeline intact.
The Time Patrol sends an agent back to each of those six dates to keep history the same.
The Time Patrol series comes out of the Nightstalkers series, where that team becomes the Time Patrol. Here is the order of books: