The Pandora Principle (Star Trek: The Original Series #49)

A Romulan Bird of Prey mysteriously drifts over the neutral zone and into Federation territory. Captain Kirk and the crew of the Enterprise investigate, only to find the ship dead in space. When Starfleet orders the derelict ship brought to Earth for examination, the Enterprise returns home with perhaps her greatest prize.
But the Bird of Prey carries a dangerous cargo, a deadly force that is soon unleashed in the heart of the Federation. Suddenly, the only hope for the Federation's survival lies buried in the tortured memory of Commander Spock's protégé, a cadet named Saavik. Together, Spock and Saavik must return to the nightmare world of Saavik's birth -- a planet called Hellguard, to discover the secret behind the Romulans' most deadly threat of all...