Yesterday's son

Yesterday's Son is a novel by the American writer A. C. Crispin, published by Pocket Books in 1983. It is a Star Trek novel, and it and its sequel make up what the author called the Yesterday Saga.

The story begins with an attack. The Romulans want the Guardian of Forever, the time portal that Federation scientists have been studying on the planet Gateway, and the Enterprise is ordered to defend it or prevent it from falling into the wrong hands. Spock's interest in the situation is personal: while studying the archaeological records of the world of Sarpeidon, which no longer exists, a scholar has found a cave painting from its ice age that shows a Vulcan face. Spock understands what it means, because he once travelled to Sarpeidon and knew a woman there, and he uses the Guardian to go back for the child he left behind. He arrives at the wrong point in time and finds a grown man rather than a child, which changes the nature of the meeting he has come for.

The novel was the first Star Trek book other than the film novelisations to reach the New York Times bestseller list, and it was followed by Time for Yesterday.