While the Doctor endeavors to amend putting Romana in great peril, she experiences a foretaste of Skonnos's fate on the ravaged planet Crinoth, and a shocked but undeterred Soldeed gets a glimmer of how he's been used by the Nimon.
Another man is found dead, strangled by the Doctor's scarf, so the Doctor is put on trial. By some of the archaic rules of law, he's made to "prove" his innocence by becoming a test subject in a new tachyon experiment with time. The result will prove his innocence or guilt, so they say.
With the saboteur and murderer revealed and arrested, peace should now be restored, but a new threat emerges - a war-monger who's both ready and eager to aggress upon other worlds with a newly manufactured warrior army.
The Doctor, Romana and K-9 arrive on the forest planet Tygella, where the native Tygellans now live in a underground city, due to the carnivorous plants. Where the Doctor finds he has been wrongly accused of stealing the Dodechedron, the city's power source. Only to discover the culprit is Meglos, the cactus-like last survivor of the desert planet Zolfa-Thura and he bids to return the Dodechedron to Zolfa-Thura for his own evil purpose and plans to use it as a powerful weapon and he plans to wipe out Tigella.
With the TARDIS occupants trapped in a chronic histeratic loop (forever reliving the same moment in time), the devious Meglos - disguised as the Doctor - gains the trust of the Tigellans along with full and private access to their power chamber.
With the dodecahedron gone, the city goes power critical. Its death, imminent. With Romana seeking escape from Meglos' pirates, and Meglos seeking escape from the sealed-up city, and the Doctor himself seeking to understand what's been going on (and why everybody wants him arrested), religious factions move in to take over the city.
With the Doctor about to be ritualistically sacrificed to appease the god Ti for stealing the dodecahedron, Meglos and the pirates return to Zolfa-Thura with the dodecahedron to fire up an ancient weapon and blast Tigella out of existence.
In answer to a summons from the Time Lords, the Doctor heads the TARDIS for Gallifrey where he must return Romana, but having tasted adventure and seen foreign worlds, she's none too happy about the return. Their journey, however, is interrupted by a rare space/time phenomena that lands them - not on Gallifrey - but on Alzarius. There, humanoid inhabitants begin to encounter an infrequent but regular cycle of perilous weather, called Mistfall, that coincides with the emergence of dangerous creatures.
As the humanoid community takes refuge in their crashed spaceship, venomous marsh spiders begin to hatch from the local fruit while bestial marshmen emerge from the waters to walk the land. It is quite apparent that the marshmen desire entry into the spaceship. Meanwhile, as a curious marshchild shadows the Doctor, a group of teen rebels decides to take over the TARDIS.
Infection from a spider bite causes Romana's personality to change. Meanwhile the Doctor, after discovering that the spaceship society is in a perpetual state of repair and upgrade, has morality issues with the ship's leading scientist who readies himself to perform gruesome experiments on the live marshchild while it's fully conscious.
Marshmen invade the Terradonian ship thanks to Romana, who seems to be in communion with them. While the ship's inhabitants flee for their lives, the Doctor discovers a crucial fact why these humanoids can never return to the planet Terradon.
Looking for scientists to help them find a way out of E-Space, K-9 locates a habitable planet for them to check out. They find a tower harboring royals overlooking a single village of grubby peasants, no fauna other than bats, and a band of rebels hiding out from a severe law that prohibits learning.
The Doctor and Romana are escorted to the tower to meet the royal leaders, hoping they can find answers to why this society evolves backwards. The Doctor recognizes the tower as an old Earth spaceship, and further finds that the old fuel tanks, far from empty, are full - but with blood. Meanwhile, Councilor Aukon senses intelligence in Adric and selects him to be the their first "Chosen One."
The Doctor finds archive material in the TARDIS that tells him he's dealing with an ancient enemy of the Time Lords, a nearly indestructible king vampire who's ready to revive and reenter normal space, spreading death, once again, across the universe.
The Doctor joins the villagers' revolt against their vampire rulers, unsure how to kill the king vampire. Meanwhile Adric is first in line to join the new vampire order while Romana is laid out as a sacrificial Time Lord for the awakening king vampire and his swarm of bats.