The one where the Doctor does a full sci-fi free historical drama. Also the first proper "lost serial".
A long historical adventure, in which the Doctor and his companions join Marco Polo on his journey into Cathay. Polo sees the TARDIS arrive and believes it to be a magical box, so he decides to take it to Kublai Khan in Peking in order to win imperial favour. He's desperate to go back home to Venice, and hopes that the gift of what he believes to be a Buddist meditation device will convince Khan to finally let him go home.
The TARDIS still hasn't recovered from the previous adventure, and she's closed off much of her facilities (including the food and water machines) to repair herself. This proves to be a problem when Tegana, a villainous diplomat travelling with Marco, empties the caravan's water supplies.
The heroes find themselves forced to accompany Marco and his caravan on the long, long trek through the desert. Susan befriends a teenaged girl named Ping-Cho, Marco's ward, who's on a journey to go marry a wealthy 70-something man, entirely against her will. Ian and Barbara get along well enough with Marco, although Marco reluctantly acts as their jailer. The Doctor isn't in the mood to play nice, and simply tries to repair the TARDIS and sneak off.
After being introduced to Kublai Khan himself, the Doctor manages to befriend the old emperor and eventually foils Tegana's plot to overthrow him. In the confusion of the battle, the gang steals the TARDIS back — although the Khan notes that he would have been kind enough to give her back anyway. Marco gets to go home three years later, and Ping-Cho's fiance conveniently turned out to be Too Dumb to Live.
All seven episodes are missing, making it the earliest serial to have any missing episodes at all as the first three serials have survived. Several fan-made reconstructions, attempts to "recreate" missing episodes using telesnaps and other sources of visual material along with the full existing soundtracks, have been made, including one in colour. The serial's audio has also been released on CD by the BBC, with linking narration by William Russell (Ian). An abridged, 30-minute reconstruction is also available on the DVD set "The Beginning", on "The Edge of Destruction"'s disc.
- Artistic License History: In this serial Nogai Khan was the head of the Golden Horde, and at war with the Yuan Dynasty led by his archrival and kin, Kublai Khan. in actuality Nogai Khan had no animosities towards Kublai Khan. It was his nephew, Kaidu Khan, that was his sworn nemesis and antagonist for many years, synchronous with the events of this serial.
- Achilles in His Tent: The Doctor.
- Arranged Marriage: Ping Cho's storyline. She's 16, her husband-to-be is well over 70. She's not happy about it, and the Doctor gives Marco a grand What the Hell, Hero? for condoning the marriage.
- "Ass" in Ambassador: Tegana spends most of the serial attempting to murder Marco and undermine Kublai Khan's court.
- As You Know: "Where is the key?" "The Doctor has it. You wouldn't let him come up here."
- Beethoven Was an Alien Spy: The TARDIS crew travel with Marco Polo and meet Kublai Khan.
- Book Safe: Marco hides the TARDIS keys in a secret compartment in his journal.
- The Emperor: Kublai Khan, who's actually pretty accommodating and reasonable towards the travellers.
- Exact Words: Ping Cho promised Marco that she would not tell the TARDIS crew where the keys were hidden. She did not promise that she would not take them herself.
- Field Trip to the Past: The TARDIS crew's adventure with Polo and company spans months, making this one of the longest Who stories in terms of real time spent.
- Future Slang: Susan lets some slip when talking to Ping Cho.
- Hero Antagonist: Marco only holds the TARDIS crew ransom because he desperately wants to return to Venice.
- Historical Domain Character: Marco Polo, Kublai Khan.
- Home Sweet Home: Both Ping Cho and Marco's main motivations. In the end, Ping Cho accepts the Khan's offer to stay in his court while Polo is allowed to return back to Venice after 18 years.
- I Never Said It Was Poison: Tegana knows about a passageway in a cave he claims he has never visited.
- Kneel Before Zod: The Doctor is ordered to kowtow before Kublai Khan. He refuses, but not out of pride he's an old man and very stiff from having just walked most of the way across China. Fortunately, the great Khan, who is also quite elderly, finds the scene amusing.
- Lighter and Softer: When the story begins, the Doctor is still his original "angry old man" type. After being absent for an entire episode, he returns as his more widely-recognized "grandfatherly" type.
- No Name Given: The mercenary bandit is never named in the story, and is only named Kuiju in closing credits.
- Obviously Evil: Tegana wears his heart on his sleeve regarding his hostility and distrust towards the TARDIS crew.
- Portrait Painting Peephole: The Doctor and Susan enter "The Cave of Five Hundred Eyes" which has walls painted with two hundred and fifty men. Susan sees one set of eyes moving and screams.
- Professional Gambler: Kublai Khan and the Doctor. The latter tries winning the TARDIS back after already winning pretty much half of Asia. He loses.
- Scenery Porn: Based on the surviving publicity photos and telesnaps, the story must have had some truly exceptional sets and costumes.
- Shown Their Work: John Lucarotti did an immense amount of research in writing this serial. He mainly drew from Marco Polo's diaries.
- Travel Montage: Marco narrates portions of the journey via voiceover while a map of the route is shown onscreen.
- What Year Is This?: The Doctor asks Marco what year it is, leading Marco to ask the Doctor exactly how long he has been travelling.
- Written-In Absence: As a result of William Hartnell's illness, the Doctor spends all of the second episode inside the TARDIS.
- Yellowface: Just about every actor in Khan's court except Zienia Merton, Ping-Cho's actress, who is half-Asian.
- Your Princess Is in Another Castle!: The travellers have outwitted their hosts/captors and regained control of the repaired TARDIS. Then, as Susan is about to join the others inside, Tegana grabs her from behind.