It is the second novel in his Galactic Center Saga, and continues to follow the scientist Nigel Walmsley, who encountered an extraterrestrial machine in the previous book, In the Ocean of Night, aboard an expeditionary spacecraft, searching for life.

In 2061, the Lancer arrives and discovers a primitive race of nomads, broadcasting using organs adapted to emit and receive electromagnetic radiation (hence "EM"s).

His analysis reveals that their technologically advanced civilization had attracted the attention of machines, who attacked with orbital bombardments that levelled the EMs' cities and cracked open the crust of the planet, permanently altering the ecosphere.

His ideas are regarded as being too speculative; the consensus is that Watchers are simply a form of weaponry left over from the suicide of biological races, and the Swarmer invasion is a grab for a new world.

News comes from Earth (delayed nine years by the speed of light) that the Swarmers have begun land invasions; the tense superpowers each suspect the other, and escalate into a full-scale nuclear war.

The following section is from the second edition of the book to bridge over to the continuance of the Galactic Core Saga: The Watcher is eventually blinded by being coated with a life-form native to the moon, which eats metals and other such materials, thus allowing the humans to board the ship.

Dave Langford reviewed Across the Sea of Suns for White Dwarf #58, and stated that "a fat, impressive demonstration that one can do ultra-'hard' SF with every rivet placed just so, and still write well.