Eleven light-years away, a new deep-space observatory finds the impossible: an exact duplicate of Earth, appearing eleven years behind.

At first, every event matches history.

Scientists watch disasters before they happen. Governments see wars, elections, scientific breakthroughs, and strategic mistakes long before they unfold on the other world. The discovery becomes the most valuable intelligence advantage in human history.

Then Earth Minus One begins to change.

A plane that crashed here lands safely there. A war never begins. People who died on our Earth remain alive. Others exist there who were never born here.

The duplicate is no longer repeating the past.

It is diverging.

Dr. Katherine Calder is among the first scientists to realize the telescope has done more than observe another world. It has activated an ancient connection between the two Earths—one that allows Earth Minus One to receive glimpses of our present.

We believe we are watching our past.

They believe they are watching their future.

As governments fight to control the information flowing between worlds, Katherine discovers something far more personal on the other Earth: the daughter she lost eleven years ago is still alive there.

But every new difference between the two planets is making the connection unstable.

The two Earths were never meant to remain separate forever.

At some point, the system will reconcile them.

Only one history may survive.

Now humanity faces an impossible question: if warning the other Earth could help it avoid our worst mistakes—but also make it the world chosen to continue—do we save them anyway?

Because when the experiment ends, there may still be an Earth.

Just not two.