Novelisation
The episode was adapted in the first Year One Space: 1999 novel Collision Course by E.C. Tubb, published in 1975. Tubb would make several changes to fit the story into the narrative of his novel: (1) As this story immediately follows 'Collision Course', Retha would appear from behind Atheria after that world's disappearance; (2) Carter and Sandra were romantically involved after her previous (unseen) break-up with Morrow; (3) The temporally-transformative mist was not confined to one area; when Koenig fired into the ground to frighten away whatever was watching him and Helena, mist rose up from the hole blasted in the soil and engulfed the pair.
Read more about this topic: The Full Circle