It is said in the Bible that no one can serve two masters.

It’s an even worse idea to trust two Masters, but the Doctor has no choice because all of them must fend off a growing Cyberman invasion. He must also deal with the end of his incarnation.

What results is a touching end to what has been one of the finest seasons in years, but not quite the end for our favorite Time Lord.

SPOILERS BELOW

Fans have been wondering how the series would handle the exits of Peter Capaldi as 12 and showrunner Steven Moffat, and it will be tough for the upcoming Christmas episode to match up to an episode that has brought out incredible surprises along with sadness and a satisfying ending for Bill Potts (Pearl Mackie).

The Doctor has to deal with Bill as a Cyberman, and the Master (John Simm) who was actually responsible for that. It’s bad enough that he and his female future form Missy (Michelle Gomez) seem to be smitten with each other, but thankfully not too much. Anyway, the Master says that after he crashed into the spaceship he was able to turn it into a Cyberman factory. He clobbered the Doctor, who was able to covertly change some programming so that Cybermen would go after Gallifreyans, too.

Master and Missy try to take a shuttle craft to escape, though Cyber-Bill prevents that. That’s how she’s able to bring the Doctor to floor 507, which looks like a nice rural countryside. Floor 507 has scarecrows that move, which are actually Mondasian Cybermen who have tried to invade so they can “upgrade” the kids.

Bill is shown to us as herself in a barn, as she wonders why everyone is scared to look at her. She soon discovers what has been done to her. Moffat does a smart thing to alternatively show her as herself and as a Cyberman, similar to how they showed Oswin and her real form in “Asylum of the Daleks”. It’s a tense and tragic scene with both Mackie and Capaldi doing it very well. Bill even says she doesn’t want to live this way, but the Doctor is not giving up. After all, Cybermen don’t cry but she did.

The relationship between the Master and Missy is pretty strange. When she knocks him out to help the Doctor, she feels the lump on her head. When he mentions a part on his TARDIS that broke when he crashed into the space station, she “reminds” him how a scary girl convinced him to always carry a spare. The pair make a perfect time paradox, and it’s pretty funny. If things were a bit different, they’d make a great Time Lord/Companion combination.

Meanwhile, Nardole has come up with a plan to fool the Cybermen into thinking the kids and adults have lots of weapons. It starts with one of the kids tossing an exploding apple. That way, the Cybermen will change their plans, long enough for the kids to get to another part of the spaceship.

The Doctor hopes the Master and Missy will help, or at least be able to, for once in both of their lives do something kind. Of course, they’d rather split in their TARDIS, but at least Missy thanks the Doctor for trying. Is that a hint?

As they all seem to be ready to leave in the elevator, Missy hugs the Master…and stabs him. That never happened between Doctors, that’s for sure. As she’s about to stand with the Doctor, the Master kills her (or is it himself) with his sonic screwdriver.

It’s just weird, right? Still, it’s quite a sendoff for both of these actors who played pure evil in their own way.

The Master finds it ironic that he’d stab himself in the back by killing his future self, but some fans suspect it may not be the last we see of them…or shouldn’t be.

So, it’s just the Doctor and Cyber-Bill against the Cybermen, while Nardole, reluctantly, has to bring the children five floors above. At least there’s one woman who would be happy to see him stay and help her take care of the kids. The Doctor’s plan works, but Bill’s Cyberman programming overtakes her and she fires her ray at him. His last words are “I am the Doctor, the original, you might say” (from “The Five Doctors”). Cyber-Bill survives the blast, but she stands above him and weeps. She may be a Cyberman, but she can still cry.

And that leads to what could be a deus-ex-machina plot twist, one that viewers have cried over.

Remember Heather (Stephanie Hyam), the pilot from the season opener? She finds Bill through her tears and turns Bill just like her. Bill could be human eventually, but why not travel the universe for a while? Before that, though, they get the Doctor into his TARDIS. A tear from Bill drops on him, and she hopes someday, they will meet again. Her last words, “When there’s tears, there’s hope”, which were the last words from the third Doctor in “Planet of the Spiders.” Then Bill and Heather leave, and maybe have a bite to eat at Ashildr and Clara’s TARDIS Diner (we hope).

The tear actually revives the Doctor, and as he regenerates he uses his force of will to stop the process. He vows he will not change again.

Actually, that would be a good thing. Peter Capaldi has been one of the most provocative, and political, Time Lords in history. Why else would he say that sewage, smartphones and Donald Trump are inevitable?

However, regeneration is also inevitable. Soon Twelve will leave,  first spending time with another Doctor…the third version of the original, you might say.

Coming this Christmas, the First Doctor will be played again by David Bradley, who played William Hartnell in “An Adventure in Space and Time” four years ago. Looking at this, the original may wonder how this incarnation came about.

By the way, Capaldi will have a farewell appearance at Comic-Con in San Diego later this month, just as David Tennant and Matt Smith did. Expect Hall H to be packed.