The Mark of Archanon

A survey below the moon's surface reveals a stasis chamber that the Alphans were unaware of. The chamber is found to contain two aliens, a man and a boy, in suspended animation. Once revived, the man identifies himself as Petrec and the boy Etrec and says they are emissaries from the planet Archanon. As Etrec is befriended, Helen discovers the two have a shared genetic disorder that may spell doom for everyone at the moonbase.