Doctor Who season 14 cast in "73 Yards," taking the audience through an entire alternate timeline that was ultimately scrubbed from existence in the episode's final moments. It was a wild ride, but it left a lot of unanswered questions.

Ruby's timeline in "73 Yards" begins with the Doctor's disappearance, and when she attempts to look for him in the TARDIS, she finds herself unable to gain access. She's ultimately left with no choice but to leave the blue box on the Welsh cliffside and return home. Although she does revisit the TARDIS much later in life, the Doctor's sentient time and space machine never goes back on its decision to keep Ruby out - that is, at least, until the new timeline. Interestingly, the TARDIS may very well have had a very understandable reason for its actions.

The Doctor's TARDIS Knew The Old Woman Was A Future Version Of Ruby Sunday

The TARDIS Has A Unique Way Of Perceiving Time

The TARDIS doesn't perceive time in the same way as the franchise's humanoid characters, and it also has additional insight into temporal matters that many other figures in the Doctor Who universe lack. With this in mind, it's very possible the TARDIS was aware of the fact that the gesticulating old woman who remained 73 yards away from Ruby Sunday throughout the episode, was actually a future version of Millie Gibson's character.

Ruby doesn't discover the truth about her semper distans stalker until the end of "73 Yards," when she's transported to the hillside and sees her younger self arrive in Wales with the Doctor.

Ruby doesn't discover the truth about her semper distans stalker until the end of "73 Yards," when she's transported to the hillside and sees her younger self arrive in Wales with the Doctor. The TARDIS, however, may have known about Ruby's future as soon as the old woman appeared. Although the future that led to Old Ruby being there at all was ultimately wiped out by her own actions, upon her initial arrival in Wales, growing old and appearing 73 yards away was still her destiny. The TARDIS could easily have been aware of this and taken preventative action.

What Could Have Happened If The TARDIS Had Allowed Ruby To Enter

Ruby's Death Away From Earth Would Have Created A Paradox

In some sci-fi properties, paradoxes are temporal events that, while unfortunate, can often be folded into the plot and generally accepted as part of the story. In the Doctor Who universe, paradoxes are not taken lightly. Paradoxes in Doctor Who have the potential to end the universe, so it's understandable why they're taken so seriously.

If the TARDIS had permitted Ruby to enter when she tried to board near the beginning of "73 Yards," it could have been the start of a reality-shattering paradox. Knowing this, the TARDIS took a harsh but necessary step to avoid such a disaster. Even without the Doctor, Ruby might have found a way to pilot the TARDIS if she'd been allowed to board.

"73 Yards" was written to for Ncuti Gatwa's absence due to his involvement with the final season of Sex Education​​​​​​.

If so, it's possible she could have found herself stranded even further from home and possibly on a different world altogether that's filled with potential life-threatening hazards. If she dies on a distant planet long before she reaches the age of her older counterpart, she'd never have become the woman on the hill. Assuming Old Ruby is at least partially connected to the Doctor's disappearance, then her absence also means Gatwa's character would have remained.

In short, Ruby entering the TARDIS only to end up never having to enter the TARDIS at all is impossible, which makes it a paradox.

Therefore, Ruby wouldn't have had to try and enter the TARDIS to look for him. In short, Ruby entering the TARDIS only to end up never having to enter the TARDIS at all is impossible, which makes it a paradox. Even if the TARDIS didn't consciously know all the steps that could lead to this paradox, a strong enough feeling of temporal unease may have been enough for it to decide to keep Ruby out.

Doctor Who Will Probably Never Officially Reveal Why Ruby Couldn't Board The TARDIS

The Mystery Of "73 Yards" Is A Huge Part Of The Episode's Appeal

"73 Yards" is arguably Doctor Who season 14's best episode, and there's a very good reason for that. The story is so enigmatic and compellingly unclear that it allows for endless discussion without ever arriving at an explanation that can be reasonably confirmed to be 100% correct. By intentionally leaving so many aspects unanswered, such as Ruby's inability to board the TARDIS, the intrigue remains intact, and more narrative possibilities remain alive.

It would be very rewarding if writer/showrunner Russell T Davies one day decided to share the official reason why Millie Gibson was denied entry to the blue box, and whether it was even a conscious decision by the TARDIS at all. The paradox explanation is just one theory to explain the Doctor Who scene, but the real reason could be something completely unexpected.

Key Background

• DID YOU KNOW: 73 Yards was the first episode of the fourteenth season to be filmed.
• It's also the highest rated episode of the season on IMDB (8.2)