4×21 March 10, 2000
THE ARMAGEDDON VARIATIONS (Serial 57)
Setting: Seattle, WA, UNIT-time
Regular Cast: Hugh Laurie (The Doctor), Sarah Michelle Gellar (Lizzie Thompson)
Guest Starring: Jonathan Frakes (Agent Blackwood), Denis Forest (Malcolm), John Lithgow (John Manning), Rodger Bumpass (Voice of the Morthrai Council)
In voice-over, the Doctor describes the “Monty Hall” problem, in which the participant is asked to choose one of three doors, behind only one of which is a prize, then offered the chance to switch his selection after one of the other doors is opened. The Doctor explains the problem in terms of parallel universes. His explanation is wrong about a key aspect of the problem, but he reaches the unintuitive correct solution: switching doors increases the chances of winning. The Doctor and Lizzie have just arrived back in the present at UNIT to find the Morthrai mothership approaching Earth. Agent Blackwood pressures the Doctor to give UNIT Time Lord technology to combat the aliens, but the Doctor, having witnessed the dire consequences of humanity obtaining such technology, refuses, insisting that they attempt a diplomatic solution. He uses the TARDIS’s communications equipment to request an audience with the Morthrai ruling council. An attache from Washington accompanies The Doctor, Lizzie, and Blackwood to the mothership. The humans argue that, despite the Morthrai’s technological superiority, human numerical and resource advantages will ultimately lead them to a military victory. Given the great cost to both sides in a military confrontation, the Doctor suggests a compromise: human bodies which are near death or suffering from severe brain injuries could be given the the Morthrai as hosts, and with their technology and increased hardiness, the Morthrai could live in areas of the planet uninhabitable by humans. Even Lizzie is taken aback at the possibility of sharing the planet with aliens, but the Morthrai leadership seem to be satisfied with the terms. Negotiations come to an abrupt halt when multiple nuclear launches occur on Earth, targeting not the mothership, but other Terrestrial nations. Malcolm contacts the council, informing them that he has secretly launched a coordinated infiltration of dozens of nuclear installations across the Earth to throw the planet into chaos and neutralize its military capabilities. With no further need to negotiate, the council prepares to execute the Doctor and the humans… And the Doctor is back at UNIT, moments after emerging from the TARDIS. This time, the Doctor advises Blackwood to launch an immediate attack on the mothership before it can send reinforcements to Malcolm’s contingent. Missiles eventually destroy the mothership, but not before it can launch a retaliatory bombardment which kills billions. UNIT itself is attacked by Morthrai soldiers, and the Doctor realizes that the aliens were able to evacuate their mothership before its destruction. Blackwood sacrifices himself to buy the Doctor time to locate Malcolm’s base. Before he dies, the Doctor reveals that he is using his Time Lord abilities to play out possible timelines in order to find a way to defeat the Morthrai. This is dangerous, because two points determine a line, and thus, anything he witnesses in two different timelines becomes “locked in”. On the next reset, the Doctor again pushes for negotiation, but this time as a delaying tactic, sending the attache while remaining behind himself. Though UNIT is able to defeat Malcolm at his base, the mothership launches a surgical strike which disables Earth’s nuclear capabilities. The Doctor tries many more variations, and despite his efforts, more details get locked in. He eventually realizes that the attache is a deep cover Morthrai agent who, left unsupervised, will give the ruling council key strategic information. Since he is now committed to sending the attache, on the next loop, he sends Lizzie along with him. Once Malcolm is defeated, the Doctor and Blackwood travel to the mothership via TARDIS. The Doctor tries to offer the council the same deal as before, but the attache turns on them. Blackwood dispatches him and reveals an explosive device with which he intends to destroy the mothership. Abandoning Blackwood in disgust, the Doctor tries to flee with Lizzie, but the TARDIS refuses her entry. Unable to escape with Lizzie, the Doctor resets the loop a second before detonation. Explaining that he’s locked in too much of the timeline to change his approach, the Doctor allows the timeline to play out almost exactly as before, but this time, he leaves Blackwood behind to deal with Malcolm and joins Lizzie on the mothership early. When the Morthrai council refuse his offer of a diplomatic solution, the Doctor reveals that he has sabotaged their weapon systems, leaving them defenseless against human counterattack. Advising them to leave, he and Lizzie prepare to depart. As the TARDIS still won’t allow Lizzie inside, he prepares to send her back to Earth with the mothership’s teleporter, but Manning, who like Malcolm, has become obsessed with the glory of conquest, tries to shoot the Doctor, hitting Lizzie instead. As she dies in his arms, the loop resets again. Before Blackwood can even ask, the Doctor volunteers to use Time Lord technology to help the humans defeat the invaders.