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The starship Marco Polo is on route on a routine mission to the planet Rossum. While the crew of five is in hibernation, strange things begin to happen. A sort of outline of a human being moves around on the monitors in the control room and puts the cameras out of commission. This could only mean one thing, a stowaway. The crew discovers they are not alone, even though the computer only registers five people on board.

The crew is split on the question on how to find the stowaway. At the same time, Marco Polo is threatened by another situation. The starship's destination, The Nexus Station, fires a torpedo against the ship. This is due to the fact Nexus cannot risk that the stowaway reaches the station. The torpedo can be stopped, but only after the stowaway has been found and captured. The crew's morale and relationship is put to the test as time is running out and death is imminent.

The crew makes use of technology and logic to find the stowaway, but it is a hopeless situation as the biological template is a perfect copy of the host the creature copied. This calls for a solution where the crew must improvise and make the creature reveal himself by confusing the biological template of what makes a human being.