Who is Ruby Sunday? The ultimate 'Doctor Who' fan theory

There's never been a Doctor Who season arc like Season 14's mystery of Ruby Sunday (Millie Gibson), companion to the 15th Doctor (Ncuti Gatwa).

Since she was introduced in "The Church on Ruby Road" — set on her 19th birthday, Dec. 24 2023 — fans of this foundling have wondered who her parents were.

Doctor Who has thrived on season-arc mysteries since its return to our screens in 2005. From Rose Tyler (Billie Piper) and "Bad Wolf", through the controversial "Timeless Child" reveal in the Jodie Whittaker era, there's always a thread to keep us guessing amidst stories that are, otherwise, largely self-contained.

Russell T Davies, veteran Who showrunner, has form for tying these mysteries up in a satisfying manner, and they can potentially lead to spin-off shows (as with season 2's multiple mentions of an organization called Torchwood, an anagram of "Doctor Who.")

Here's everything we've gathered on Ruby so far, from "Church on Ruby Road" to the latest episode, "73 Yards."

Ruby is a foundling, left by a mystery woman.

"The Church on Ruby Road" opens with the Doctor witnessing a woman in a cloak leaving a baby on the steps of a church in Manchester at midnight on Christmas Eve 2004. It is snowing. The Doctor watches the woman walk away. Tears are streaming down his face.

Viewers presume this is the same scene as at the end of the episode, where the Doctor goes back to 2004 to save baby Ruby from time-traveling goblins. But as we found out in "Space Babies," the next episode, the Doctor's memory of it can change. In its second version, the woman in the cloak raises a finger to point accusingly at the crying Doctor.

Does that mean he is connected to Ruby's family somehow? Could that be why the Doctor mentioned his granddaughter, Susan — who hasn't really been a presence in the show since 1964 — in the following episode?

Ruby can make it snow.

"I have been to the ends of time and back, and I have never seen anything like this before," the Doctor says in "Space Babies." He's referring to the fact that real snowflakes appeared on the space station as soon as Ruby said the word "snow."

The snow happens again in "Boom," at the exact time that the Susan Twist-voiced AI ambulance is supposed to read out the name of Ruby's "next of kin." The AI starts glitching. "It never snows here," says one marine. "No, it's Ruby," says the Doctor. "This has happened before."

It also snows in "73 Yards," though it's unclear whether Ruby is making that happen. "I used to be able to make it snow," she laments at one point.

Ruby keeps meeting other foundlings.

Though she has her foster family, mother Carla Sunday (Michelle Greenidge) and grandmother Cherry Sunday (Angela Winter), who refuses to move from what is now the family flat in London, Ruby keeps running into other foundlings. Lulubelle, another foster kid born on Christmas Eve, was the first. Then came the Doctor, who revealed that he too is a foundling (fact check: after the events of the "Timeless Child" arc, this is true).

But that was just the beginning.

The babies in "Space Babies," arguably the Maestro in "The Devil's Chord" (who thanks the Doctor for getting rid of her father, The Toymaker), famously motherless children John Lennon and Paul McCartney in that same episode, the freshly-orphaned child in "Boom": For whatever reason, the TARDIS keeps bringing Ruby Sunday to people who have lost a parent or two.

Speaking of which ...

Is Ruby connected to Rose Tyler?

It wasn't Ruby's birthday, Christmas Eve 2004, when The Doctor (David Tennant) met Rose Tyler for the first time, chronologically speaking, visiting at the end of his regeneration in "The End of Time" (2010).

But it was one week later exactly: New Year's Eve, 2004. It was snowing. Rose, Davies' original companion, is still technically "Bad Wolf" — a complex space-time event with shards scattered throughout the Doctor's history. And Rose also lost her father, Pete, as explored in the first season story "Father's Day."

Are these events connected? Could the showrunner be aiming to tie Rose in one more time? That would certainly give new fans a reason to go back to the 2005 season of the show, starting "New Who" all over again, while they wait for Ncuti Gatwa's second season in 2025.

Ruby Sunday's origin: More fan theories

Ruby is identified as homo sapiens in a DNA scan by the TARDIS at the end of "Space Babies," so bang goes any fan theories that suggest she is a Time Lord like the Doctor, or has Time Lord DNA.

But that doesn't mean we're without threads to pull on.

One big thread is the "Carol of the Bells," a popular Christmas tune playing throughout "Church on Ruby Road" (and one that's actually based on a Ukranian New Year song — more grist for the Rose theory!) The Maestro identified this tune as a "secret song" in her soul that prevented Ruby's musical spirit from being absorbed.

"There's a definite answer," Davies has insisted in multiple interviews. The penultimate episode of the season, dropping on Disney+ June 15 and called "The legend of Ruby Sunday," looks set to reveal it.

How to watch: New episodes of Doctor Who drop every Friday night at 7:00 p.m. ET on Disney+, where available, and simultaneously at midnight on BBC iPlayer in the UK. The season finale, "Empire of Death," airs June 22.