It was a dark and stormy night in Lake Geneva, 1816. Known as "The Year without a Summer" after Mount Tambora's eruption screwed with the global climate, this was the summer that inspired Mary Wollstonecraft Shelley (Lili Miller) to write Frankenstein. So perhaps it should not be surprising Doctor Who's cold open begins during the infamous night in June she spent in Switzerland with Lord Byron (Jacob Collins-Levy), John Polidori (Maxim Baldry), and Claire Clairmont (Nadia Parkes). (The Tales of the Dead they're reading is a real novel, published in 1813.) Nor should it be a surprise this is the night the Doctor drops in.
Ryan: Nobody mention Frankenstein. Nobody interferes.
Yaz: And nobody snog Byron.
That Graham doesn't know Jane Austen from Mary Shelley is merely the tip of the iceberg. The Doctor knows this is a delicate but pivotal moment in history, but she wants the fam to have a chance to "soak up the atmosphere." Whatever she was expecting, I doubt it was silly dancing and gossip now that Mary's baby William is in bed. As this is a history episode, it gives the show an excellent opportunity to drop facts about Mary's non-marriage to Shelley at the time and Byron's growing scandals. Also, let's hear it for that long-suffering valet, Fletcher (Stefan Bednarczyk), who also plays the piano.
But something is off. The Doctor's psychic paper doesn't work, for one. Percy Shelley, Mary's not-quite husband, is missing. And strange things happen, like vases tossing themselves into walls, skeletal hands jumping out of paintings, and Graham suddenly discovering he's going round in circles. Ok, maybe that one wouldn't be so strange, except he went up the stairs only to find himself down again. William's nanny Elise (Sarah Perles), is terrified. Some of the goings-on have rational explanations, like Clairmont trying to break into Byron's study. (She hopes to read his diary and see if he's actually in love with her.) But the ghosts do not.
Byron's taken with the Doctor, ready to snog at a moment's notice. (Considering that historical women from Mme de Pompadour to Elizabeth I have thrown themselves at male Doctors in the past, that's only fair play.) She tells him she is here to visit Mary, but they might be staying longer, as something is very wrong with the house -- it's evil. Then there's Polidori, a depressive sort who will go on to invent the entire vampire genre. (1819's The Vampyre, look it up.) He gets cranky and challenges poor Ryan to a duel. But the skeletal hand shows up (it's from Byron's collection) and dives right at Ryan's throat, disrupting those plans.
When asked if anything else strange happened, Mary admits Percy saw visions before becoming indisposed. She agrees to take Yaz and Ryan to visit him, but they get caught in a stairwell loop. And when the Doctor says she needs to get out of the house, she too gets stuck. Meanwhile, Graham sees Polidori fall into a sleepwalking state, and walk through a wall. Polidori's new ability makes the Doctor realize the walls aren't there. The house is a perception filter. By closing their eyes, Mary, Yaz, and Ryan break the loop and get to William's nursery, where he's been replaced with more of Byron's skeleton pieces. The Doctor finds the door to the outside, only to discover an invisible forcefield surrounding the house.
But at least they can see outside the house, and the vision Byron was having, someone floating above the lake. The Doctor realizes it's a time traveler, trying to break free of something. Just then, a "ghost" shadow finally solidifies inside the foyer.
It's a Lone Cyberman (Patrick O'Kane). An angry, unfinished Lone Cyberman. (He looks kinda borgy, to be honest.) And it wants to know if she's the Guardian.
The Doctor goes after the Cyberman, demanding no one follow, lest she lose another companion to these monsters. (Visions of Bill's fate dance before us.) The Cyberman makes quick work of the valet and nanny, though he doesn't kill William. As the Doctor questions the new visitor, trying to ascertain why it's here and what it's after, Yaz points out the Doctor's orders didn't say "stay in the room," which means they can split up and find William and a way out. She, Mary, Ryan, and Byron find a room covered with symbols Percy drew on the walls. Graham, Polidori, and Claire head to the cellar to see if there's an escape hatch.
After being shocked with lightning a couple of times, the Cyberman starts to communicate, reciting Percy's latest writings. Meanwhile, Graham stumbles across Percy (Lewis Rainer) down in the cellar. He apologizes and says he's the Guardian. Percy accidentally let a glob of the Cyber AI, known as Cyberium, take hold of him one day while out walking. It's been hiding him ever since, distorting people's perceptions of both him and the house. The symbols he drew all over the walls are messages from the Cyberium, and he keeps seeing the history of the Cybermen as they burn and pillage across the galaxy. It's also killing him. The only way to save Percy Shelley is to let the Cyberman have what it wants and pull the Cyberium out of him.
Ryan tries to argue the Spock position, the good of the many outweighing the needs of the few. But this isn't Star Trek; it's Doctor Who. Shelley's place in history matters. He's not just one man; he's an influencer who's work affects generations to come. Remove him from the timeline, and the world changes. The Doctor will not have it. The Cyberman comes for Shelley, and though the Doctor tries to take the Cyberium inside herself to protect it at the last minute, she fails. The Lone Cyberman has gotten what he wants. The Earth is saved, and Mary Shelley has a new inspiration to write her famous novel. But at what cost?
The Doctor says Step Two of the plan is to clean up the mess she made in Step One. But the fam shouldn't come with her; she'll drop them in 2020, and go alone. Yaz pulls out her drawings she took from Percy's room, the symbols the Doctor needs. The Doctor needs her. The Doctor needs them, and they're coming with.