The following contains spoilers for “The Legend of Ruby Sunday.”

In an episode full of misdirection, the biggest one has to be its title, given we’ve learned very little about what Ruby Sunday’s legend actually is. Instead, the first part of the series’ two part finale is essentially an hour to build a sense of dread that spills over in its final moments. I could cheat and say “The Legend of Ruby Sunday” is just “Army of Ghosts” — the first half of the 2006 season’s finale — with a bigger budget. Except the big bad that reveals itself at the end is a villain from a far deeper cut than the usual corners of Doctor Who’s history.

The Doctor and Ruby arrive at UNIT HQ to ask about the mysterious woman — Susan Twist — following them around the universe. UNIT, meanwhile, has been monitoring someone named Susan Triad, a British tech billionaire who will announce her gift to humanity later that day. Even the goofballs at UNIT work out that S.TRIAD is an anagram of TARDIS and the Doctor thinks Triad, or the mysterious woman more generally, could be his granddaughter.

But there’s also the matter of Ruby’s parentage to uncover, giving the Doctor a reason not to just confront Triad. The Doctor, Ruby and a UNIT soldier enter the time window — a low-grade holodeck — to try and see who left Ruby on the steps of the church. But the history’s a bit wonky, and Ruby’s faceless mother — unlike what we saw in “The Church on Ruby Road” — turns and ominously points toward the TARDIS. Not long after, the TARDIS is engulfed in a black cloud of swirling evil that nobody’s sure what to do about.

The Doctor then meets Triad just before she gets on stage, prompting her to remember all of her other selves. Whenever Triad dreams, she’s somehow aware of those myriad alternate selves. And while she takes to the stage, the Doctor asks the team at UNIT HQ to scan the TARDIS. It is similarly engulfed in an invisible cloud of malevolent stuff that’s threatening everyone in the area.

Bad Wolf / BBC Studios

[ASIDE: This is the second time in four years that Doctor Who has tried to parody an Apple Keynote. And this is the second time that they’ve totally misunderstood how to stage one that looks even remotely evocative of what they’re parodying. I know the conventions of the tech keynote have mutated since the Steve Jobs era, but they’re not even trying.]

A UNIT staffer, Harriet Arbinger (Wait… H. Arbinger?) starts muttering about a dark prophecy while Triad goes off script. The Doctor, standing close by, watches as she turns into a skeleton monster while the TARDIS is menaced by a giant animal head surrounded by Egyptian iconography. Turns out Susan isn’t the Doctor’s granddaughter, or even a key component of the story, but an innocent. An innocent who has been co-opted by Sutekh, an all-powerful Egyptian God we first saw in 1975’s “Pyramids of Mars.” Cue the credits.

It’s a slender synopsis, mostly because these scenes are played slowly as the tension ratchets up. “The Legend of Ruby Sunday” takes its time, letting the screw turn gently until you’re almost happy when the big reveal happens. It’s a gripping ride on a first watch, although I imagine it’ll not have too much value when you go back to it a third or fourth time. But, then again, that’s often been an issue with episodes penned by Russell T. Davies. It’s also a good way to juice bookings for next week’s finale which will get a UK cinema release on June 21.

Was it easy to guess that we’d be getting Sutekh back after his one outing in “Pyramids of Mars?” The rumor mill certainly pulled in that direction over the last month or so, and it’s not as if we didn’t get a clue or two along the way. Longtime Davies fans will recall that Vince watches the part one cliffhanger at the end of the first episode of Queer as Folk. And we’ve already had a whole scene from “Pyramids of Mars” lifted — the jump into a ruined future — in “The Devil’s Chord.”

Bad Wolf / BBC Studios

If you are unfamiliar, “Pyramids of Mars” is a classic, and another blockbuster from the pen of the series’ best 20th century writer, Robert Holmes. At the time, Holmes was the series’ script editor and had commissioned a story from writer Lewis Griefer. But Griefer’s material was so poor that Holmes and producer Philip Hinchcliffe decided a replacement was needed. So Holmes was tasked with writing a whole new episode in a tiny amount of time. The finished episode was credited to pseudonym Stephen Harris, but it’s all Holmes under the hood. Sadly, because of various rules around writing credits, “The Legend of Ruby Sunday” end credits actually give credit to Lewis Griefer as Sutekh’s creator and omit Holmes, which feels pretty rough.

But that one minor injustice aside, let’s bring on the finale.

Susan Twist Corner

• Well, looks as if we have our answer that Susan Twist was something of a misdirect.

• Gabriel Woolf, who voiced Sutekh in 1975, is back to give voice to him now.

• When Mrs. Flood was left to look after Cherry, she was clearly aware of Sutekh’s return and seemed delighted by it. But she didn’t appear to be a harbinger, so it’s likely she’s representing another, different malevolent character from the series' past.