John Seton, a marine pilot in World War II, had crashed his airplane. Physically he healed well. He couldn't adjust to civilian life. His engagement to Marcia didn't even hold his interest. Dozing off in his armchair, John is awoken by a figure identical in appearance to himself. The figure explains he is Aard Calgon, a scientist from the year 3951. He can project himself into the past if he can find an exact look-alike for himself. Aard can change places with John, and asks if they might for a while. Neither person is to reveal the exchange.