After nearly 60 years, Doctor Who finally gets around to visiting a historical figure which one would have thought would have turned up already: Nikola Tesla (Goran Višnjić). The Serbian-American inventor was the father of A/C current, best known for his electrical and mechanical engineering inventions and his futurist view on the world. He's also famous because of what he's not credited for: The lightbulb. It is popular lore Telsa invented the lightbulb first. He just couldn't capitalize on it, and then Thomas Edison beat him to the punch and became synonymous with electricity.
Graham: "Don't worry, this ain't our first rodeo."
Ryan: "We've never been to a rodeo."
Graham: "You're not helping, Ryan."
With such a background, one would have thought The Doctor would have popped round to see the guy before. And a quick dive into Whovian lore shows Telsa is a favorite of Seventh and Eighth Doctor, but only in radio plays and books. This is the first time the Doctor has crossed paths with him on screen.
When the episode opens, Tesla is at Niagara Falls, trying to raise money for his "Niagara Falls generator along with his (real-life) loyal secretary Dorothy Skerrit (Haley McGee). But his efforts are undermined by, among other things, his claims of signals he received from Mars. (It's true Tesla spent his life trying to communicate with Mars, though he never succeeded.) Unfortunately, there's also a dead body, one that looks to have been killed by electric shock, which will really undermine his investors' confidence.
This dead body is only the first of many. When Tesla and Skerrit happen across a floating metal ball that glows green, they suddenly find themselves chased by someone shooting at them with a weapon that's not a period-appropriate gun. Thankfully, that's when the Doctor bursts in, takes one look at the situation and advises both to run. They follow her back to where she left Ryan, Yaz, and Graham, who are playing cards on a midnight train to New York City.
Yaz and Ryan have no idea who Tesla is, a reminder of how he's erased from modern history. (Ryan asks if he has something to do with the cars.) Throughout the hour, the show will go on to list out all of the modern inventions Telsa dreamed up first (and his very steampunk names for them), as well as recounting his biography. But right now, there's little time for introductions, as the alien gunman is back again. The Doctor has his gun, but no matter. This time he's shooting red lighting from his hands and has glowing red eyes.
Once back in New York City (Brooklyn, back when rents were reasonable), at Tesla's place, he presents the glowing ball to the Doctor. She says it's an "Orb of Thassor" one of many out in the universe sent out by the ancient race, in hopes of spreading their knowledge long after they were gone. But it's been repurposed as a tracking device of some kind, which explains why someone is hunting it down. But who? Telsa suspects Edison (Robert Glenister), whose spies are literally taking photos from the fire escape, affording the Doctor, Graham, and Ryan the chance to pay a visit to his famous rival.
Edison at first claims he knows nothing of aliens, but one look at the gun that the Doctor took from the red-eyed man, and he's suddenly willing to see them without an appointment. Not that he knows anything about the weapon; he's just mistaken the Doctor for an inventor with a sales opportunity. But it's a good thing Edison invited the Doctor in, as the red-eyed man has arrived, making short work of the employees before attacking Edison and the Doctor as well. Edison may be a businessman first, but he proves himself enough of a scientist to follow the Doctor's thinking in containing the creature.
Back at the apartment, Tesla enlists Yaz to help him tap into the energy source within the orb. But it's a siren call to the creature, who slips the Doctor's trap. But it didn't come for the orb. The objective was to capture Telsa, who they transport away to the awaiting ship along with Yaz. It turns out, like the clockwork creatures who once hunted Mme De Pompadour back in "The Girl In The Fireplace," these beings, called the Skithra, believe he is the key to fixing their ship.
With things escalating so fast, The Doctor rounds up the TARDIS, hustles Edison and Skerrit aboard, and gets to work figuring out where Tesla and Yaz are. After locating the ship, she transports herself over only to discover it's a cobbled-together mess. The Queen of the Skithra (Anjli Mohindra) insists the ship is hers, but the Doctor recognizes it as Venusian technology with a Klendov warp drive. The Skithra are scavengers, and now they've scavenged Tesla because he was smart enough to intercept their signal and signal back.
Returning to the TARDIS affords the show a chance to let Tesla have his "The internal dimensions transcend the external" moment. (Never. Gets. Old.) Using his current Niagara project, which is basically WiFi, but for electricity, the Doctor and Tesla plan to shock the Skithra ship out of the sky, killing the Queen, which will take down her red-eyed minions. Edison poo-poos that it could work, but naturally, it will. Edison and Yaz head out to get everyone off the street, where Edison's uncanny ability at salesmanship and fearmongering about Tesla's experiments has people racing for their apartments.
The Queen almost foils the Doctor by traveling down to Earth herself, threatening to lay waste to Earth and all the people on it. "Have you ever seen a dead planet?" she asks, reminding the audience of Gallifrey's current state. But it's a short-lived distraction. Soon enough, she's back on her ship, and Telsa is sending lightning into the sky to drive the Skithra away, leading tomorrow's papers to dub this "Nikola Tesla's Night of Terror." If only they knew he saved the Earth during it.