While surveying a cavern for crystals to maintain the Eagles'
air scrubbers, a team of Alphans discover a capsule containing two humanoids in
suspended animation – a man and a boy. The capsule bears a mysterious six-pointed mark. The aliens are extracted. Unseen by the Alphans, the man awakens and reaches for a
pick-axe, the mark blazing on his forehead. However, on hearing the boy is safe, he loses consciousness and the mark vanishes. Later reawakening in Alpha's empty Medical Centre, the man, Pasc, rouses the boy – his son Etrec. The boy begs his father to kill him so he never receives the mark, but Pasc cannot bring himself to do it. Pasc tells the Alphans that their people, the Archanons, outlawed violence and dispatched emissaries throughout the cosmos to teach this philosophy. He was the leader of a team who were sent to Earth and driven to mutiny by the brutality they found there. The other Archanons, including his wife Lyra, forced them into stasis. In Medical, Dr Russell identifies a virus in both Archanons – dormant in Etrec, but active in Pasc. Elsewhere, technicians open the stasis chamber's power unit and reactivate the transceiver that Pasc used to record his command log. Fearing that the Archanons will use the transceiver to track them down, Pasc tells Etrec they must flee Alpha. While attempting to steal an Eagle, Pasc captures Russell. In response, Controller Verdeschi seizes Etrec and holds him at gunpoint. Each man threatens to kill his respective hostage, but Pasc knows that humans are too weak to kill in cold blood. Etrec reveals that the virus is a neurological pathogen known as the "killing sickness". He elects to stay on Alpha while Pasc is given an Eagle in exchange for Russell's life. Finding the Eagle's controls deactivated by command override, Pasc threatens to kill Russell unless he is granted safe passage off Alpha with Etrec. During the confusion, Russell blurts out a message that her assistant, Dr Nuñez, interprets as an order to take blood samples from the Archanons. Carter discovers Etrec in the throes of the virus, his forehead burning with the mark. Attempting to excise it, the boy stabs his own head. Meanwhile, an Archanon spacecraft approaches the Moon and transmits a message to Alpha warning the crew that they are in danger. Etrec draws attention to the transceiver recordings, in which Lyra explains how Pasc contracted the virus and killed two of their team. As the disease is often passed from father to son, Pasc and Etrec were put in stasis together until a cure could be found. Using Etrec's blood, Nuñez devises a
serum to cure the virus. However, Etrec has suffered fatal blood loss. At Russell's urging, Pasc has his blood transfused into his son, saving Etrec but at the cost of his own life. Maurna, an Archanon woman aboard the ship, is granted entry to Alpha. She explains that the Archanon body cannot produce blood quickly, so no Archanon could give the amount needed to prepare a serum and survive. Pasc quips that the sickness grants him the privilege of suicide, and dies. Maurna takes Etrec into her care and the Archanons leave Alpha. ==Regular cast==