While Avon and Vila attempt to steal a T-P crystal from Fosforon (with the help of Tynus, an old "friend" of Avon's), Blake and Cally worry about a 700-year-old ship which has been picked up. Their worries are confirmed when a plague breaks out on the planet.
The Federation have recently changed their communications systems and the crew need a new special crystal which decodes the Federation's messages. They head for the planet Fosforon - home of one of the Federation's communication stations.
Avon and Vila teleport down to the surface to meet up with an old acquaintance of Avon's - Tynus and Avon were part of a massive fraud operation. Avon was arrested and kept quiet about Tynus. Avon is now calling in that favour - Tynus now works in the control centre monitoring the communications systems and is ideally placed to help Avon and Vila.
Meanwhile, the Liberator spots a vessel homing in on the planet. The Wanderer class ship appears adrift and doesn't respond to the crew's calls. Cally detects human life aboard, something malignant, but the Federation base has already launched a salvage ship. Blake sends a coded message to the planet, warning them of the danger aboard the drifting ship.
Avon and Vila persuade Tynus to help them, arranging for the crystal to appear to breakdown. A replacement will be ordered, a small fire will be started and in the confusion Vila and Avon will make off with the original crystal. But Tynus arranges for a message to get to Federation Command, betraying Avon and Vila.
Dr. Bellfriar, the base's chief medical officer, ponders Blake's warning, issuing quarantine instructions around the base. Orac has determined the ship is K-Forty-Seven a vessel that went missing over 700 years ago. Blake teleports down to the base to find out what is on the ship and meets up with Dr. Bellfriar.
A human body is found onboard the ship and transferred to the base for an autopsy. But during the procedure, the body comes to life, killing the doctor. The assistants rush in to help, but on their arrival, the body is once again dead.
Blake reasons that the body has been surgically altered by some alien lifeform. They body had a scar on its skull, possibly a brain implant, which sent impulses to the body, bringing it to "life". Once its program was completed, the body was discarded. Soon the assistants who went to help are dead; killed by a potent virus delivered by the dead body.
The planned fire goes off, causing mass confusion in the communications area. Avon and Vila slip in and after a short while, Avon manages to grab the required crystal from the computers. Once back in Tynus' quarters, Vila discovers the message sent to Federation control, requesting help from Servalan.
Avon kills Tynus and creates an explosion in the communications centre, hoping to hide their theft of the crystal. Avon and Vila then teleport back to the Liberator.
Blake teleports back to the Liberator and puts Orac to use, trying to find the cause of the virus. Eventually he does and Blake provides the information to Dr. Bellfriar, hoping he can stop the spread of the virus. It turns out that the virus attacks anyone who has been into space, effectively barring space travel for humans.
Leaving the planet to its woes, Blake decides to leave a plague marker in orbit, even though Servalan is on her way. Avon tries to stop him, knowing that the plague will kill Servelan, but Blake is unwilling to risk the rest of Federation space with the virus. The beacon is left behind as the Liberator leaves orbit.
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