In the future, the spaceship Marco Polo is returning from a mission from the planet Rossum inhabited by robots(who have taken control over the planet when mankind failed to take responsibility).... The robots have no intention of being discovered and place a stowaway in the form of a biomat(artificial human being) in the ship. While the crew is having an artificial rest during their long voyage home, the biomat kills one of its crewmembers and take its place... After seeing the footage and discovering broken cameras, they discover the ingredients of a human being has been going through a meat grinder and out of the ship... As there are 5 people only in the spacecraft, the computer states that the stowaway must have killed him/herself - only there is no way to commit suicide since the meat grinder has to be maneuvered by a "human" hand.... It dawns of them that one has been replaced by the biomat - but which one? And their home - the station Nexus - refuses to receive them since they all know by now that the planet of Rossum will stop at nothing to prevent discovery... So a torpedo is sent to eleminate the ship of Marco Polo... They implant radiotransmitters on each other under the supervision of computers in order to reveal the identity of the biomat when it may fail to "act" human... In addition they come across a spaceship - who also paid a visit to Rossum and was terminated by... a biomat..... "WHICH ONE OF US IS THE MURDERED ONE?" Some music by the great Pogo https://www.youtube.com/user/Fagottron