8th episode of second season, 32nd overall. Written by Lew Schwartz (his only episode). Directed by Charles Crichton. Filmed concurrently with "The Rules of Luton".

Plot

Cmdr. Koenig and Maya are on Eagle 1, on a survey mission for at least two days. Tony is in temporary command.
Alan and Andy "Bluey" Johnson are scouting in the Catacombs, when they uncover a force-field covered cabinet bearing a Flammon ("Death Glow") symbol, with two intact bodies inside, which is later exposed during further rock collapse. The Alphans feel compelled to rescue the two males, one adult and one child inside.
They later awake, the alien man explaining he is an Archanon ("Peacebringer"), a race name Maya recognizes from legends. Pasc claims a prior, long-ago attempt to visit Earth failed because Earth's inate violence got to the Archanon team and mutiny occurred.
However, the Alphans have access to the control / "power unit" of the cabinet. Pasc is subtly evasive about its individual controls, and avoiding Medical with the intent to steal an Eagle, and it becomes clear something is wrong with them, for Pasc has urges to kill, and Etrec knows this.
Helena discovers Pasc has some active virus, and Etrec a dormant form.
Pasc plots to get away, making one attempt that fails with no great notice. A later, more violent attempt starts. Etrec is unwilling to participate in Pasc's escalating violence, despite signs the urge is starting within himself too.
Eventually, the Alphans discover a visual log system on the control/power unit, revealing Pasc and Etrec were not imprisoned due to mutiny led by Lyra (Pasc's wife and Etrec's mother), but that she, distraught, had to leave them there due to their contracting Killing Sickness, uncurable.
Pasc kidnaps Helena and commandeers Eagle 3, but cannot take off and threatens her life. Etrec, however, not wanting the Killing Sickness, injures himself trying to remove the symbol of it, and Pasc thinks Etrec is dying, and finally surrenders to give Etrec a blood transfusion that can save his life and remove the sickness.
Earlier, the Alphans unknowingly activated a transmitter on the power unit, and an Archanon ship arrives, one of them (its captain or commander?), Maurna, urgently requesting landing, which after Tony gets an explanation that jibs with recent incidents, and he grants permission.
This dooms Pasc, however: Archanons cannot lose more than a very small amount of blood before their physiology is fatally compromised, which an arriving Archanon leader, Maurna, reveals is the reason the Killing Sickness cannot be cured, because the only known means is just as fatal.
Pasc dies, and his body, and the now-cured Etrec are will return to Archanon.

Details

The Catacombs are being scouted for dylenide (crystals). Reason unknown (?), but presumably yet another of the resource needs Alpha has.
Tony and Helena both recognized the geometric symbol on the statis chamber as the Flammon -- the "Death Glow" -- which they saw on an earlier mission to Crom II. See Flammon for more detail and speculation on this and in regard to both the Croms and Archanons.
Alan befriends Etrec, including teaching him some football(?). Etrec is distraught that Pasc seemingly kills (?) Alan, and chooses to stay with Alan and the Alphans than go with Pasc when the latter holds Helena hostage to (successfully) get to Eagle 3.
Other than some brief communications, John (pilot) and Maya (apparent co-pilot) are seen little. They are frequently in danger from a meteor storm, and Eagle 3 was put on standby, on a pad, in case the two got in trouble. This made Eagle 3 most susceptable to Pasc's attempt to steal transport off Alpha, using Helena as a hostage to get to it, and threatening her life to try to get the Eagle released for launch.
Etrec uses his father's actual name at all times, not "Father" or "my father." (Note: Maya has mixed references, sometimes using father/Father and sometimes Mentor.)
Yasko is Tony's main operative in Command Center.
Maurna is already aware of the name of Moonbase Alpha as her ship arrives; but it is entirely possible her ship intercepted earlier communications between Alpha and Eagle 1.