Neanderthals have developed a radically different civilization on a parallel Earth. A Neanderthal physicist, Ponter Boddit, accidentally passes from his universe into a Canadian underground research facility. Fortunately, a team of human scientists, including expert paleo-anthropologist Mary Vaughan, promptly identifies and warmly receives Ponter. Solving the language problem and much else is a mini-computer, called a Companion, implanted in the brain of every Neanderthal. A computerized guardian spirit, however, doesn't eliminate cross-cultural confusion; permanent male-female sexuality, rape, and overpopulation are all alien to Ponter. Nor can it help his housemate and fellow scientist back in his world, Adikor Huld, when the authorities charge Adikor with his murder.