5 member crew of starship Marco Polo wake from hypersleep to find mysterious entity stowed aboard. 20 hours left to discover who was replaced by the intruder before reaching their destination Nexus or be destroyed to avoid contamination.

The spaceship Marco Polo is returning from a mission at the newly discovered planet Rossum. While the five members of the crew are in deep sleep a mysterious shape is captured on one of the surveillance monitors. Awakened the crew soon discover that one of their number has been killed, and something is living among them in the shape of a crewmate. But who is it??—Anonymous