Senior co-administrator of the Norcoast Salmon Research Facility, Dr. Mackenzie Connor was a biologist who studied the spawning habits of salmon. Then, last season, just as she and Dr. Emily Mamani were starting their research, they were interrupted by the arrival of Brymn, the first Dhryn to set foot on Earth.

And suddenly everything changed for Mac, Emily, Brymn, the human race, and all the member races of the Interspecies Union. Base was attacked, Em was kidnapped by the mysterious Ro, Mac nearly lost her life when she and Brymn escaped to the peaceful, isolationist Dhryn home world. But the Dhryn are in the process of abandoning it, hurtling out on an unfathomable path of destruction through civilized space.

As the enemy destroys life on planet after planet, the IU organizes a secret Gathering of every being with information on the Dhryn. With the data available there, Mac begins to suspect that the Dhryn's actions may be a response to something simpler and more deadly—the imperative to migrate. But even if Mac is right, can she and her team discover why this is happening and how to halt the Dhryn?