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troperville

That is the dematerialisation control, and that over yonder is the horizontal hold. Up there is the scanner, those are the doors, that is a chair with a panda on it... Sheer poetry, dear boy! Now please stop buggering me!

— The Doctor shows Steven around.

With Vicki and still-sceptical stowaway Steven in tow, the TARDIS arrives in early 1066 on the eastern coast of England. Exploring, the Doctor finds a wristwatch and a toaster, and is trapped while investigating the sounds of chanting from a ruined monastery. Quite surprisingly, the chanting was a gramophone record.

Turns out that one of the Doctor's (as yet unnamed) own people is here, complete with his own TARDIS — whose chameleon circuit actually works. He plans to defeat Harald Haadrada's Viking fleet with nuclear bazookas, allowing King Harold to win the battle of Hastings, and then help the English accelerate their development so that they have "jet airliners by 1320" and Shakespeare's Hamlet can premiere on TV.

After thwarting his plans, the Doctor removes the Dimensional Control from the eponymous Meddling Monk's TARDIS, shrinking the interior to the same size as the exterior (a coffin) and leaving the monk stranded.

The story has a number of firsts — the first Time Lord other than the Doctor and Susan (though the race is still not named); the first pseudo-historical story mixing real history with sci-fi elements and of course the first story without any of the Doctor's original companions. It also has a second, as in the Meddling Monk being the show's second reoccurring enemy after the Daleks.

Watch it here.

Tropes

Bad Habits: The Monk is not really a monk.
Compound Interest Time Travel Gambit
Crew Of One
Steven: You know, I'm beginning to like the idea of being a crew member on a time machine.
Vicki: A crew member? You'll be lucky! He's the crew — we're just the passengers.
Distaff Counterpart: The Meddling Monk receives one in the 2012 novelization of Shada. Among a list of Time Lords who broke with traditional Gallifreyan inactivity is "The Interfering Nun".
Finale Credits: The season ends with the three time-travelers' faces superimposed over a starfield.
Giving Radio To The Romans
Horny Vikings
I Never Said It Was Poison: The Meddling Monk (who is holding the Doctor prisoner) claims that he hasn't seen the Doctor but gives himself away by knowing what the Doctor looks like without Steven having told him. Subverted because Vicki and Steven soon realise that the Monk made his "mistake" on purpose to lure them inside.
The Middle Ages
Portal Slam
Ripple-Effect-Proof Memory: Discussed.
Stick Em Up
Temporal Paradox: Also discussed.
Utopia Justifies the Means
Wham Episode
Wham Shot: Vicki and Steven (and the audience) see the interior of the Monk's TARDIS for the first time.