Review of all 4 parts:
This adventure involves Tegan coming under the control of the Mara, who had remained dormant in her mind since their adventure in the previous season (Kinda) where they had previously taken her over. They influence her to get the Doctor to take the TARDIS to Manussa to attain the 'great crystal' which can bring the Mara back into physical form.
Janet Fielding as Tegan gets a really strong story where she can show her dark, possessed side as well as her struggle to fight the mental effects of the Mara. She does really well. Nyssa is given a prominent role too but I enjoy her less in this adventure. Peter Davison is strong as The Doctor and support cast Martin Clunes, Colette O' Neil,John Carson and Johnathan Morris are all good too.
The story starts interestingly and is all done well with some menace, some fun and some decent ideas. The first two parts are the strongest. Parts 3 and 4 are a little more muddled and the resolutions are slightly disappointing to me but it is still an enjoyable, slightly dark serial overall.
The effects are average but the filming is pretty good with director Fiona Cumming doing well. Christopher Bailey's script is decent and not dumbed down.
My ratings: Parts 1 & 2 - 8.5/10, Parts 3 & 4 - 7/10. Overall - 7.75/10.