The Doctor is a peace-loving, wise-cracking, pacifist Time Lord from the planet Gallifrey, but, in previous lives, she's also been known as "The Destroyer of Worlds," "the Beast," and "The Oncoming Storm." She contains multitudes, to say the least.
Just because the titular hero of Doctor Who is mostly all smiles, the Doctor can be downright ruthless when crossed. And in Season 12, Episode 4, "Nikola Tesla's Night of Terror," the 13th Doctor (Jodie Whittaker) channels some hardcore ruthlessness worthy of David Tennant's 10th Doctor. In one pivotal scene, the Doctor references one crucial test she gives her enemies before showing no mercy.
**Spoiler Warning: There are spoilers ahead for Doctor Who, Season 12, episode 4, "Nikola Tesla's Night Of Terror."**
Based on trailers, many fans believed that the spidery Racnoss from "The Runaway Bride" was making a return in this episode, but it turned out that the scorpion-like aliens were known as the Skithra. And although their Queen (Anjli Mohindra) looked a lot like the Racnoss, this was, in fact, a different alien species altogether. That said, like the Racnoss and another David Tennant enemy — The Sycorax — the Doctor was fully prepared to kill every single one of the Skithra to protect Earth.
Toward the end of the episode, the Skithra Queen taunts the Doctor, implying she clings to "the desperate hope I'll change my mind." But the Doctor fires right back.
"No, we're way past that. I gave you your chance... a chance to evolve. But you were too stupid to take it."
And, a few scenes later, the Doctor tricks the Skithra Queen, teleports her back up to her ship, and — with the help of Nikola Tesla (Goran Višnjic) — fires a 19th-century death ray at the Skithra mothership. We don't see the ship actually explode, and it appears that they decide to warp out of orbit before getting totally fried, but the intent is clear: The Doctor will vaporize you don't listen to her. But, like her previous incarnations, she'll give you just once chance before throwing the switch.
Perhaps the most dramatic example of this from a previous Doctor comes from David Tennant's very first episode, "The Christmas Invasion." After winning a sword fight with the Sycorax ruler, the 10th Doctor gives that invading species a similar option, leave the planet now, or face the consequences. When the Sycorax ruler tries to rush the 10th Doctor from behind, he ruthlessly throws a satsuma mandarin at a control panel, releasing a trap door, and sending the alien to his death.
One second prior, the Doctor had been laughing and joking; the next second, he was deadly serious. "No second chances," the 10th Doctor said, as the Sycorax plunged to his death.
Perhaps the 13th Doctor is most similar to the 10th Doctor in this one specific way. Even at the top of the episode, she goes from deadly serious to friendly and then back again. This kind of emotional multitasking is part of what gives the character of the Doctor their charm, but it also belies just how deadly they can be if you don't listen to that patented-Doctor warning. The Skithra Queen asks the Doctor if she's ever seen "a dead world," which, of course, seems to reference the fact that this season, we've already learned that a new version of the Master has burned the Doctor's home planet, Gallifrey, for an as yet, unrevealed reason.
So, what will the Doctor do when she finds out the hidden secret of the Time Lords — will she give them a chance to change? It wasn't so long ago that the Doctor singlehandedly destroyed all the Time Lords in the Last Great Time War. And, ahead of the launch of Season 12, Jodie Whittaker told SYFY WIRE that this season would reveal "all sides" of the Doctor and that she would have to deal with "challenges" she hadn't in Season 11. Meaning, if the Time Lords — or anyone — don't listen to her again, the 13th Doctor could be just as deadly as the Master.