(Photo: BBC America) 
Has everyone recovered from "Legend of the Sea Devils" yet? It’s fine if you want to take a little longer to buckle your swash and pack your action away for another day, but for everyone else, let’s talk about this this nugget at the end:

As you can see, there’s a LOT to unpick here, and as it’s the Thirteenth Doctor’s last hurrah, everything feels very significant. Shall we dive in?
1. “No regeneration, no life”
The quote that starts things off comes from the final episode of Doctor Who: Flux, in which Time, taking the form of the Doctor, warns the Doctor that “Nothing is forever. No regeneration, no life. Beware of the forces that mass against you. And their Master.”
Naturally, the forces are represented by the Doctor’s principal foes, the Daleks and the Cybermen, and the Master is represented by, well, the Master. Is it relevant that Sacha Dhawan has cut off his beard? Only time will tell... except she hasn’t.
2. The Lone Cyberman

Last seen shrunk to a tiny size by the Master, apparently this battle-damaged Cyborg is on the way back into the Doctor’s life, or the Doctor is on the way back into his (time travel: it’s fiddly).
The pristine Cybermen behind him suggest he may have found fresh reinforcements, and it’s worth noting that they are not the Time Lord/Cybermen hybrids of “The Timeless Child”. He also does not have a beard.  
3. Tegan
The lady telling her phone that it’s been “nearly four decades” since she heard from the Doctor is Tegan Jovanka, former air stewardess (cabin crew, but in the 1980s) and travelling companion of the Fifth Doctor. She met the Fourth Doctor by accident, wandering into the TARDIS while looking for roadside assistance when her car broke down on the way to her first shift at Heathrow Airport.
Very much in the Donna Noble mold as a companion who acts as the Doctor’s down-to-earth conscience, Tegan overcame possession (“Kinda”), regression (“Mawdryn Undead”) and a constant struggle to keep her uniform clean. She temporarily left the TARDIS when it left her behind by accident, and when she returned, it was after she had left air travel behind her. 

She eventually grew weary of the chaos left behind in the Doctor’s wake, and left the TARDIS after the events in “Resurrection of the Daleks”. She has made her return to active service via innumerable Doctor Who spin-off media – books and audio plays. In parallel to this, Janet Fielding, who plays Tegan, has fast become one of the most active and popular former cast members in Doctor Who fandom.
4. Ace
And the lady replying that it has been three decades for her is Ace, former problem child, bomb enthusiast and tough-as-old-boots companion of the Seventh Doctor. If Tegan is a precursor to Donna Noble, Ace is somewhere between Rose Tyler – all street-smart and bratty – and the soldier tough Martha running around with Mickey and biffing Sontarans on the probic vent. After her time in the TARDIS, she put her considerable talents to use creating her own charitable foundation, A Charitable Earth (ACE for short).
And while she may have not spoken to the Doctor in a long time, her spirit of adventure is far from idle, as this trailer from the 2019 reissue of Doctor Who Season 26 sums up rather well:

5. “This is the day you die”
Let’s be honest, the Master says stuff like this a lot. In fact, one of the first exchanges between the Doctor (Third) and the Master (First) takes place in “Terror of the Autons” from 1971, and it goes like this:
Master: “I hope I'm not interrupting anything important”
Doctor: “No, no, indeed not. You've come here to kill me, of course”
And this is after the Doctor has been specifically warned by the Time Lords that the Master will “certainly” try to kill him. Some time later, a hideously burned incarnation of the Master (“The Deadly Assassin”) even claims that part of his motivation is that he “must see the Doctor die in shame and dishonor.”
Having said that, when he does manage to capture his nemesis in “Mind of Evil,” his co-capturer offers to do him a serious mischief, to which the Master replies “Mailer, all you will do is kill him. I don't want that,” which may count as mixed signals.
6. Kate Stewart
The Doctor can always make good use of an unflinchingly loyal ally such as Kate, daughter of the Third Doctor’s ‘boss’ Brigadier Lethbridge-Stewart and former head of UNIT, the United Nations alien battling taskforce.

Astonishingly, it has been ten years since Kate first barged her way into Amy and Rory’s house, investigating the odd alien cubes that had rained from the sky overnight in “The Power of Three”. That’s halfway to her screen father’s record of 20 years, from his first appearance (then ranked as a Colonel) in 1968’s “The Web of Fear” to his grand finale in “Enemy of the Bane” for The Sarah Jane Adventures.
7. The orange spacesuits

Dan’s got one and so has the Doctor, although Dan’s helmet appears to have developed a bullet hole, so maybe he should sit the next spacewalk out. Every Doctor (since Ten) has worn these iconic orange spacesuits. They made their first appearance in “The Impossible Planet,” popping up on Eleven in “Hide,” Twelve in “Kill the Moon” and now Thirteen. And on Orson Pink in “Listen” and Clara in “The Girl Who Died.” Clearly the Whoniverse only has one space tailor.
8. Vinder

Freshly reunited with his darling Bel, Inston-Vee Vinder was last seen heading off into the galactic sunset on board Karvanista’s ship. The couple (and their grumpy canine host) were looking forward to starting their family together and seeing the delights of the universe, having scoured the wastelands of post-Flux planets to try and find one another. Mind you, if the Doctor really needs some backup, badass Bel might be the better bet.
9. The Master (again)
The Master says, “welcome to the end of your existence” instead of “welcome to your death,” indicating the “no regeneration” thing is definitely on the cards. Also, he’s got a beard this time. And long hair. This plan must have taken him ages.
Wowser, that is a lot going on in one space. Are you now ready for the final adventure?