The background to Humans has Ponter Boddit happy to be back in his own world of Neanderthals. He has reunited with friends and family and returned to his life as a physicist. Yet he can't help but feel that there remains unfinished business from his trip to the parallel world inhabited by the strange, possibly dangerous people who call themselves homo sapiens. And he would like to see Mary Vaughan again.

Humans, the second volume in Robert J Sawyer's Parallax trilogy, tells the story of Ponter's second trip to our world and the opening of the portal between worlds to a few other travellers.

It is for the most part a quiet story of the deepening relationship between Ponter and Mary as Ponter continues his investigation of the human world and develops a growing interest in the preoccupation of its residents with religion.

Meanwhile, intercut scenes of Ponter in therapy on his homeworld contribute to a growing tension in the story, as the reason for Ponter's feelings of guilt is slowly revealed. At the same time, scientists are beginning to notice that there is something odd happening with the magnetic fields of both Earths.