During a tour of the TARDIS,
the Doctor and
Sarah Jane Smith are sucked into the Mandragora Helix, a swirl of living energy. While they successfully escape, a fragment of the Helix follows them, bringing them to the 15th century Italian duchy of San Marino. There, Count Frederico and court
astrologer Hieronymous covertly kill the former's brother, the Duke, in a bid for power. Hieronymous clashes with Federico's science-minded nephew, Giuliano, who Frederico conspires to kill to prevent his succession to the throne. Sarah is caught by a pagan cult while the Doctor is apprehended by Federico's forces. The Doctor unsuccessfully tries to warn the duchy of the Helix and is sentenced to death, but escapes. He finds Sarah and rescues her from a sacrifice attempt by the cultists. Their leader is revealed to be Hieronymous, who bargains with the Helix fragment for godlike powers. The Doctor and Sarah are caught by Giuliano's forces; the Doctor befriends Giuliano and deduces that the fragment picked this location due to the cult giving it a ready power base. The trio return to the temple to destroy it; the Doctor is attacked by the fragment, Frederico's guards attack Giuliano, and Sarah is caught by the cult while looking for the Doctor. used as the location for the palace courtyard where the Doctor was to be executed The Doctor recovers and saves Giuliano, while Hieronymous hypnotizes Sarah to assassinate the Doctor. She is returned to the Doctor and tries to attack him during a confrontation with Hieronymous, but the Doctor disarms her and breaks the hypnosis. Hieronymous escapes to the temple, where Frederico confronts him; Hieronymous reveals that his body has been absorbed by the fragment, leaving only energy, and kills Frederico before plotting a coup during an oncoming masque. The Doctor convinces Giuliano not to cancel it and returns to the temple, tying wire around the altar and defending himself from Hieronymous' attacks with a breast plate. The cult breach the palace and attack the masque, killing many, but Hieronymous seemingly appears and demands that the survivors instead be sacrificed. At the temple, the cultists suddenly die and the fragment is expelled from Earth thanks to the Doctor's wire. "Hieronymous" turns out to be the Doctor impersonating him; the real Hieronymous similarly perished during the pair's fight. The Doctor and Sarah part ways with Giuliano, while the Doctor estimates that the Mandragora Helix will attack Earth again around the end of the 20th century. ==Production==