The Armageddon Factor
The Armageddon Factor was the sixth and final story of Season 16. It concluded the season-long Key to Time story arc. Mary Tamm makes her final appearance as Romana I while her replacement, Lalla Ward, appears in the story in a different role. The Black Guardian appears for the first time and isn’t seen again until Mawdryn Undead.
The sixth and final segment of the Key to Time is on the planet Atrios. Twin planets Atrios and Zeos are locked in a long-running war. The youngPrincess Astra, nominal leader of Atrios, is appalled at the devastation, but the Marshal, in charge of the war, actually possesses the power. The Marshal secretly confesses to his aide-de-camp Shapp that they are losing. He is desperate for the edge that will bring victory. On instruction from an unseen entity, he leads Astra on a fool’s errand into a trap, where she is abducted and trans-matted away.
The Doctor and Romana land on Atrios, and the Doctor’s TARDIS is soon buried in rubble from a Zeon aerial bombardment. The Marshal finds them, and believes them to be spies, along with Astra’s lover, Merak. The Doctor tricks the Marshal to calling for K9, who knocks out the lights to let them escape. They find the TARDIS gone.
Merak has followed them, and they find Astra gone. K9, meanwhile, is lured away to be recycled. The Marshal is told by an unseen force, who tells him to treat the Doctor and Romana as guests. The Marshal implores the Doctor to assist Atrios. The Doctor proposes a shield that will stop the Zeons from attacking, but the Marshall insists he create a weapon for total victory.
Romana notices that the Marshal is acting under an outside influence when he accidentally shows a small device on his neck after flipping out when the Doctor runs to save K9 from the furnace. Afterwards, the Doctor asks the Marshal to send him to Zeos. The Marshal refuses. The Doctor reveals to Romana that something is probably blocking Zeos. Astra appears on a television and states that the Zeons will destroy Atrios if the Marshal does not surrender. The Doctor is trapped by two people in masks in a trans-mat.
The unseen entity tells Marshal that the war will stop, while the entity traps him in his domain. It calls itself the Shadow, who reveals that he has his TARDIS. The Shadow asks the Doctor to open his TARDIS and bring out the other five segments and then leaves. The Doctor decides to search for the sixth segment. Merak, meanwhile, has trans-matted to the Zeos. Astra is on the ship in chains, being asked by the Shadow where the sixth segment is. Romana and K9 trans-mat to Zeos, following Merak.
Shapp finds himself with the Doctor after after being trans-matted. K9 finds the commandant of the Zeon forces, which is actually a supercomputer named Mentalis. The Marshal, meanwhile, has decided to attack Zeos. They find that Mentalis has been instructed to conceal any information on Astra. The Marshal prepares to attack Zeos.
They learn that if it is destroyed, it will destroy Zeos and Atrios – a concept known as the Armageddon Factor. The computer can’t counterattack because it’s been told that the war is over. They discover the Marshal, in a last-ditch effort, is piloting the last Atrian warship to destroy the Zeon capital with a nuclear missile. The attack will set the mutual destruction in motion.
Astra is hypnotized by the Shadow and a projection of her lures Merak, and he begins falling. The Doctor is, meanwhile, trying to reinstruct Mentalis, but accidentally triggers the computer into waiting for an attack to blow itself up immediately. The computer becomes mindless by destroying its own control center.
Desperate to stop the Marshal’s attack, the Doctor uses the five segments of the Key to Time, plus an artificial sixth segment made from chronodyne, to generate a temporary time loop around the Marshal’s ship.
Meanwhile, K9 is hypnotized by a small device which takes the guise of a distress call. K9 is then transported away where he is reprogrammed. The TARDIS heads to the third planet, and the Shadow laughs that the Key to Time is his.
The Doctor, Astra and Romana are on the third planet, and each one is separated from the others. The Doctor finds another renegade Time Lord, Drax, in the corridors of the third planet. Drax was employed with threat of death, and he was forced to build Mentalis. The Doctor persuades Drax to assist him, and they remove the control device from K9. Drax repairs his dimensional circuit.
They uncover the ultimate truth: a third party, known as the Shadow, is manipulating the entire war from his enormous vessel midway between the two planets. An agent of the Black Guardian, the Shadow has been watching the final segment of the Key (Princess Astra herself), setting the war in motion and simply waiting while the Doctor risked life and limb to find the first five segments.
Drax enters with the dimensional circuit and shrinks the Doctor down.
Drax also shrinks himself, and they decide to have one of them provide a distraction. Romana, meanwhile, realizes that Princess Astra is the sixth segment to the Key to Time. Merak transports himself to the third planet. Drax and the Doctor return to normal size and take the Key to Time from the Shadow. Merak stays behind to look for Astra, who has turned into the sixth segment.
When the artificial time loop expires, the Doctor and Drax disarm Mentalis. When the Marshal fires, the war rockets are deflected by a force field, destroying the Shadow and his ship instead. Hearing his minion’s dying words, the Black Guardian disguises himself as the White Guardian, telling the dying Shadow that he will trick the Doctor into giving him the Key to Time himself.
Astra converts herself into the final segment, completing the Key to Time and giving the Doctor, for the moment, absolute power over the entire Universe. The White Guardian appears on the TARDIS monitor. He demands that the Doctor hand the Key over to him. When he rather callously dismisses Astra’s sacrifice, the Doctor realizes this is actually the Black Guardian in disguise. The Doctor orders the Key segments to disperse across the Universe, which also allows Astra to reunite with Merak. The Black Guardian is furious and threatens to destroy the Doctor. However, the Doctor has installed a randomizer on the TARDIS console, ensuring that neither he nor the Black Guardian knows where he’ll end up next.