After a season that featured history lessons, a cute monster and battles with small but important stakes, it’s about time the Doctor not only met an old enemy but dealt with a situation that may destroy the universe.
All of that and more were part of the season finale, where power and revenge play a big role.

SPOILERS BELOW

The story is based on what happens when faith is exploited by an evil entity, and that is the case of the Ux, a race of only two people but who can actually recreate realities with their minds. These two people, Andinio (Phyllis Logan) and Delph (Percelle Ascott) , literally make mountains with their minds when suddenly…someone arrives. They decide it must be the Creator, and immediately follow him.This happened 3400 years ago, and now the Doctor and her crew notice its effects.

They arrive at Ranskoor Av Kolos (or “disintegration of the soul”), a planet that gives off psychotropic waves that can distort reality. For some reason, nine distress calls have been traced there, and they investigate thanks to the help of neural balancers. They find a ship that seems to be in good shape, but a man who doesn’t remember much. He eventually remembers his name is Paltracki (Mark Addy), and he was supposed to investigate some atrocities. They may be connected to the dozens of wrecked ships all over the planet

Then, a woman’s face appears on a screen. She tells him the “Creator” demands he return something he took. The facial marking reveal she is Andinio, and the Creator also demands he listen to her.
The voice is familiar to the Doctor. It’s Tzim-Sha (Samuel Oatley)of the Stenza, who she thought was sent back to his planet. Instead, he landed in Ranskoor, and took advantage of his new surroundings. More on that later.
The return of Tzim-Sha angers Graham, and he warns her he wants revenge for the Stenzan killing his wife Grace. She warns him if he tries that, he may not be welcome in her TARDIS. She says Graham should be the better man, but he may have a point.

Once Ryan finds out, he may wonder if Graham should have revenge because Grace would not have wanted that. That’s an interesting point that serves its role in the story.

Still, what is the crystal Paltracki found, and what is inside? The Doctor decides to find out by bringing it to Tzim-Sha.

Once the Doctor is reunited with Tzim-Sha, it’s a tense meeting where she wants to know how he got there. He almost gleefully replies, implying what’s happening is her fault, too (something she doesn’t want to accept). He says the return switch she used back in the season opener got him to Ranskoor 3400 years ago. He claims he’s their God, and the Ux believe it. By combining the Ux’s ability to change things with their minds with Stenzan tech, they’ve destroyed a lot of ships…and worse.
That’s revealed by Paltracki, just after they find four more crystals. He says the crystals have planets inside.
That’s right, planets. The Ux somehow put them in crystals, and Tzim-Sha decides he wants to add another to his collection. Of course, it’s Earth, and the Ux blindly obey him.

How could the Doctor solve all this? It’s not easy, but she and Yaz use their neural balancers to get Andinio and Delph to realize what they’re doing, and they they’re worshipping a false god. They’re able to harness their abilities, the TARDIS  and Stenzan tech to put the planets back where they should be and save the Earth. That’s one major trick.

It’s not long until Graham confronts Tzim-Sha, and seems willing to kill him. Tzim says Graham is no warrior, and he’s right. Graham doesn’t shoot him, but Ryan shows up….just in time to see Graham shoot Tzim in the foot. They also come up with a better idea: putting Tzim-Sha into one of the statis chambers he used on Patracki’s crew and a few others. So, Tzim will be on ice for some time, but fans get the idea he’ll be back somehow, just like a Dalek.

Paltracki will send the prisoners back home, while Delph suggests to Andinio they should find a new home. The Doctor, of course, has the final word on what has been an unusual but very entertaining season:

None of us know for sure what’s out there. That’s why we keep looking. Keep your faith. Travel hopefully. The universe will surprise you constantly.

That’s a great way to wrap up the season, except there’s a New Year’s episode that features an evil that was swept through the centuries. It’ll be interesting if it’ll be a bridge to the next season, which won’t start until early in 2020.
One thing is certain, the “fam” will still be there.