This article contains spoilers for the “Doctor Who” episode “The Well.”

The latest episode of “Doctor Who” conjures a feeling of dread that has been missing from the show recently. Called “The Well” and written by the showrunner Russell T Davies, along with Sharma Angel-Walfall, it is gripping and eerie, featuring a monster even the Doctor struggles to understand — despite the fact that he has faced it before.

Set half a million years in the future on an inhospitable planet called 6-7-6-7, “The Well” sees the Doctor join a squadron of soldiers investigating a mining operation that has lost contact with base.

The operation’s only survivor is a cook, Aliss (Rose Ayling-Ellis). But it’s strange: People keep thinking they see something move behind her, though clearly nothing is there. The thing behind Aliss has no face, no physical form, not even a name. It’s more the idea of a monster — and only once before has such an unseen villain featured on “Doctor Who.”

At the episode’s climax, the reveal comes that some 400,000 years earlier, Planet 6-7-6-7 was called Midnight.

That name alone will be enough to delight “Doctor Who” fans. “Midnight” is a 2008 episode, created by Davies and starring David Tennant. Seventeen years later, fans still regard it as one of the show’s best — and most frightening — episodes.

But just what makes “Midnight” so terrifying? Here’s everything you need to know.

What happened in ’Midnight?’

In “Midnight,” the Doctor heads out on a bus tour across the deadly but beautiful diamond-covered planet of the same name, where intense radiation means nothing can survive. But then a loud noise bangs on the outside of the vehicle, sending the ragtag group of tourists on board into a panic.