Doctor Who '73 Yards': Who exactly is that old woman following Ruby Sunday?

The latest season of "Doctor Who" has already taken the TARDIS to outer space and 1960s London. The destination in episode four, "73 Yards," is present-day Wales, but that doesn't make things easy for companion Ruby Sunday — in fact, this spooky tale may be her scariest adventure yet.

With the Doctor nowhere to be seen, Ruby finds herself stalked by a ghostly woman wherever she goes. But where did this apparition come from, why has she chosen Ruby, and what does she say that induces such fear in everyone she speaks to? We've got (most of) the answers below. And if you need to get caught up, our Doctor Who streaming guide has tips on how to watch the Fifteenth Doctor's first full season.

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What's the old woman doing in "73 Yards?"

Not very much, aside from haunting Ruby Sunday while maintaining a constant 73-yard distance — "semper distans" (always distant), as a patron of the Y Pren Marw (The Dead Wood) pub describes it. (For everyone on the metric system, one yard is 3 feet, or just under 1 meter). Wherever the Doctor's companion goes, the woman appears to follow, though Ruby doesn't put this theory fully to the test. Instead, she decides not to get on a plane or a boat in case severing the bond kills either her or her sinister follower.

Although she makes no effort to interact with Ruby, the woman has a profound effect on anyone else she speaks to. Even a brief conversation is enough to prompt people to run away screaming and avoid all future contact with Ruby. Even Ruby's own mother, Carla, shuns her daughter after a close encounter with the ghostly figure.

For others, even a combination of psychic training, telepathic dampeners, mesmeric shielding, and necklaces of silver and salt are enough to protect Kate Lethbridge-Stewart and her team at UNIT (Unified Intelligence Task Force) from the woman's poison words.

What does she say to have such a profound effect on her "victims?"

Nobody knows, and it looks like it's going to stay that way.

"You will never know," showrunner/writer Russell T. Davies said on the documentary show "Doctor Who: Unleashed." "I'm never going to tell you what she says. It's kind of up to you to sit there and think, 'Well, what could someone say that would make a mother run away from her daughter forever?' You've got to look at yourself and think what would make me do that, and once you start to do that, you enter the real horror story, the dreadful things that are being said there."

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Why 73 yards?

At that distance, a person's face will appear blurred and indistinguishable to a viewer with normal 20/20 eyesight. In the unusual case of the woman, however, zooming in with a camera lens won't help you get a better view — even UNIT's most sophisticated equipment can't deliver better resolution than that aforementioned viewer 73 yards away.

Davies came up with the figure on Swansea Pier in South Wales, using the fact that the railings are 3.5-feet long to estimate the distance to a passer-by who was "a blur but not a blur."

"I handed the script in saying, 'That's not exactly a very scientific method,'" Davies admitted. "Turns out I was right. My scientific methods using railings on Swansea Pier were exactly right, and we've stuck with 73 yards!"

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Where's the Doctor while all of this is going on?

He vanishes as soon as he disturbs a fairy circle on the cliff top where he and Ruby have just landed. Ruby is left to fend for herself, while the TARDIS becomes a permanent monument on the Welsh coast.

That's the in-universe explanation, anyway…

"73 Yards" was the first episode filmed for this season, when star Ncuti Gatwa was still working on his previous show, "Sex Education," for Netflix. With that in mind, Davies crafted a Doctor-lite episode in the tradition of "Love & Monsters" (2006), "Blink" (2007) and "Turn Left" (2008) to ensure production could get underway without its lead actor.

Intriguingly, "73 Yards" has several echoes of "Turn Left" (also written by Davies), in which then-companion Donna Noble (Catherine Tate) found herself in a parallel, Doctor-free timeline.

Why does this happen to Ruby?

For the first couple of decades, Ruby can find no meaning in the haunting. Then, in the late-2040s, a leaders' debate for an upcoming election gives her a new sense of purpose. She recognizes nuclear weapon-loving Roger ap Gwilliam as "the most dangerous prime minister in history" and vows to "save the world."

After working her way into the new PM's organization, Ruby seizes her opportunity during rehearsals for an event at the Cardiff City Stadium, positioning herself exactly 73 yards from her target. A few whispers from the woman and ap Gwilliam's sent running, quitting Downing Street without explanation moments later.

The intervention gives Ruby plenty of satisfaction, but her elderly companion remains.

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So who is the woman?

When the episode jumps forward another 40 years, we gradually learn the truth. With an elderly Ruby on her deathbed, the woman appears in her hospital room — significantly closer than the obligatory 73 yards. As she turns to face Ruby, Ruby flatlines and her life flashes before her eyes — until she finds herself standing on those cliffs in Wales, looking out at the Doctor and her younger self stepping out of the TARDIS all those years ago. The mysterious woman was Ruby all along.

Events on the cliff top play out more or less as they did before, only this time Ruby spots the woman and hears her whispered warning: "Don't step." She tells the Doctor not to enter the fairy circle, and the Doctor-free timeline is instantaneously averted.

What causes the events of "73 Yards?"

"73 Yards" doesn't deliver hard and fast answers, though Davies suggests in a video on the Doctor Who YouTube channel that the fairy circle is the cause of Ruby's plight.

"Something profane has happened with the disturbance of that fairy circle," he said. "There's been a lack of respect. The Doctor, who's very respectful of alien cultures and alien lifeforms and alien mythologies, he's just walked through something very, very powerful. So something's gone wrong, and something has just corrected. It's like Ruby had to spend a life of penitence, in which she eventually does something good, which brings the whole thing full circle, which kind of forgives them in the end."

This tallies with Davies' previous remarks that this sci-fi show is taking a "sly step towards fantasy." Indeed, the shift is acknowledged on screen by Kate Lethbridge-Stewart, who points out that the extraterrestrial experts at UNIT are now dealing "more and more [with] the supernatural. Things seem to be turning that way these days."

"Doctor Who" streams on BBC iPlayer in the UK and Disney Plus elsewhere in the world. New episodes debut at 7pm ET/4pm PT on Fridays, and midnight on Saturdays in the UK. Our guide to watching new "Doctor Who" episodes explains more.