This is the first story to feature the Fifth Doctor and Peri and it would appear to be quite soon after Planet of Fire as the Doctor is promising to give Peri a tour of an alien world and she is still showing an interest in botany. However, the way Peri talks during the first two episodes gives the impression she has already made at least one other trip in the TARDIS. The two characters are separated quite early on and, although they communicate with each other, don’t actually meet again until the last episode. This gives Peri a lot more to do than in Whispers of Terror and the character comes across very well as a result, especially in Parts Two and Three....
