A class one medical emergency summons the Enterprise to the Federation outpost Tanis. There, a grisly surprise awaits them. Two of the lab's three researchers are dead, their bodies almost entirely drained of blood. There are no clues, no records of their research, no remnants of their work. There is only the outpost's sole survivor, Dr. Jeffrey Adams, who holds the secret of what happened at Tanis, and might now happen aboard the Enterprise.