The first line in this book is someone announcing that Earth is dead.
The star ship, an American one, all men, is held together when Donnan, the engineer, starts to order the captain about, rousing him from stupor, and goes about the ship getting those men who can to act. He speaks with Ramri, their alien pilot, and then they find that the solar system has been loaded with missles, which give them some evidence about possible culprits, and reason to mistrust it, and reason to flee. Though they have reason anyway. They head out into the cluster to find a place where they can work, and possibly find some way to contact the other ships -- some of which have women on board, which would be necessary for humanity to go on.
On a nearby planet, the crew slowly disintegrates, until the captain and first officer die. Then Donnan's speaking of what should be done puts him in command, to take charge of the question of what they are to do, how are they to find other survivors, and who is responsible. They join into the local war in hopes of finding out.
Another star ship, European, all women, finds Earth the same, and also heads out to the stars -- another cluster, and they miss the messages left in nearby star systems. They hold together better, partly because the captain orders everyone to take a tranquillizer that keeps them calm until the first shock has worn off, and find more clues in the solar system about the cause. Sigrid has an idea about how to find other humans, which is a subplot throughout the book.
The story weaves on through several worlds and alien cultures, a kidnapping, prisoners of war, a carefully wrought prop, new discoveries, and twists on who is responsible and why, and questions about humanity.