Upon reactivating the TARDIS's secondary control room,the doctor and Sarah-Jane Smith find themselves drawn into the equally dimensionally transcendental, crystalline realm of the Mandragora Helix. Narrowly escaping an intense burst of energy,the TARDIS redirects itself to late 15'th century San Marino. A midst a power struggle between Giuliano and his tyrannical uncle Count Fredrico,complications arise when the astrologer Hieronymous predicts that the Duke will be killed upon the next planetary alignment. Sarah is captured by members of the Cult Of Demnos,who plan to use her as a sacrifice to a great power that is soon to arrive. After witnessing the Mandragora energy burst flying past him,the doctor is knocked unconscious trying to rescue Sarah. There is a peasant revolt occurring in San Marino and the energy kills a peasant along the way. The doctor awakens to find himself taken in by the Counts men. Waking up in a prison cell with Sarah they pair manage to escape. The doctor concludes from what Sarah has witnessed that the Mandragora Helix is a life form intent on influencing the ancient Roman cult of Demnos in order in take hold of humanity for some unknown reason.
Giuliano's men locate Sarah and the Doctor,hoping they can join forces with him and his companion Marco to stop the bloodshed caused because of Hieronymous's superstition. Complications ensue with the Sarah is taken in by Hieronymous who hypnotizes her to attempt to kill the doctor. After she and the doctor are again captured by the Count,the doctor has understood that Hieronymous has is the leader of the cult of Demnos,who believe the Mandragora helix is their mystical overlord. Even worse it is also understood that this Helix embodies a life form intent on using the cult in order to halt human progress at this time so it can take over as the sole ruler of the planet. Trusting the doctor just enough to investigate the Doctor's claims,Count Fredico releases the doctor only to be killed by Hieronymous,now willingly overwhelmed by the Helix. As Giuliano is upped in rank to Duke he is forced into trusting the doctor's judgement in using the guise of Giuliano's indoctrination gala to force the Helx into escape by draining it's energy and leaving an extremely thankful Giuliano and Marco behind as he and Sarah continue on their journey.
This find Robert Holmes era Doctor Who story superbly blends science fiction conceptualizing with historical intrigue. The bio-kinetic Mandragora helix fears the progression of humanity as a rival force in the universe. Upon arriving it finds an Earth witnessing the end of the renaissance and the beginning of the modern era. This era itself it filled with rivaling individuals such as Giuliano and his tyrannical uncle Count Fredrico. Giuliano,a man devoted to reason and the new frontier of science confronts both his egoist uncle and the helix absorbed Hieronymous,who himself is overtaken easily due to his own fear of the unknown. This is a very telling story in terms of illustrating humanity's constant personal divisions when it comes to cultural evolution. This also marks the debut of the Victorian looking "steam punk" styled secondary TARDIS control room,which would be used for a number of Doctor Who serials after this one. The doctor also reveals,among other things,that he gave Sarah-Jane Smith his own ability to interpret foreign languages. And this adds to the deep camaraderie between the doctor and Sarah-Jane.