As Gillen mentioned in his newsletter today this is a dense issue so this is a dumbed down recap. Hopefully this will be a team effort and others can fill in the important details lol.
Mother Righteous is narrating and we learn how she magically uses the "thank you's" in various ways. Elixir saves Destiny who says she's the only one who can do anything to stop MR. MR confirms that she messed with the mutants' connection to the gates, making all the mutants want to go "to the place mutants are all kind of connected to", the White Hot Room. She herself couldn't go because she's not a mutant so she used some of the "thank you" power to create a version of herself, an extension of herself, that was a mutant. This is the MR we see in the WHR with Jean, while the real MR is puppeteering it all from her sanctum.
Xavier and Sinister arrive at Muir Island and get to Sinister's lab after easily getting past Orchis special forces. The point of this trip is that Sinister wants to review the data from the Sins of Sinister timeline as he doesn't trust MR's highlight reels.
As MR is leading a chained Jean through the WHR, we learn that after a lot of research MR decided the mutants were the key for her to reach Dominion.
Xavier gets some cool moments as he's able to reconnect Sinister's computer using the mind of a Norwegian electrical engineer.
Xavier says Stasis' and Orbis' Dominion attempts are labeled in Sinister's timelines. Each reached out to Dominion but were consumed and then Sinister reset the timeline. Sinister can't see the data and Xavier says someone has been messing with him. Xavier confirms that there have been three Dominion attempts, his attempt during the Red Diamond empire, Orbis with a mysterium/M'kraanite crystal construct, and Stasis did something terrible to the sun. He says there hasn't been a fourth attempt yet. They realize it's Mother Righteous that's going to go for it and that she must be found and stopped.
Data page of MR's plan, super simplified:
Get to the WHR via manipulation of mutantkind.
Find dead Jean (she knew the plan was for her to killed at the Gala.)
All of the living mutants in the WHR will have Jean "messed in the head" and make her pliable.
Jean will want to go to the center of the WHR where it's most peaceful, leading MR there.
She'll then sacrifice Jean and bleed her out, using Jean's connection to the infinite power of the Phoenix to make herself a Dominion.
Exodus, Hope, and Destiny reach MR but are attacks by magical monsters she creates (using the rest of the thank you's.) I have to be honest that I was viscerally disturbed to see Jean stabbed again as I'm quite over it. MR stabs her and uses her dagger to write in the snow with Jean's blood. Exodus launches Destiny to her MR she has to stop. Too late as MR has already written in the snow that she has become a Dominion and lived happily ever after. "The flesh becomes the word. Ascension." She starts to float in the air but then comes across what appears to be a sheet of paper. Someone else wrote something and the words are on fire. "However, his mistress of stories should have known there's always a twist. Look behind you." MR turns around, quietly says "no" and implodes in a PLIF of blood. MR returns to the ground with a scream saying it should have been her, but it's him. Destiny kicks her in the face which is very satisfying and says, "this is what I've been trying to tell you. You've doomed us all."
A brand new narrator: Once upon a time. A cliche of children's fiction. As if the fact it's been repeated so often means anything. Stories are pathetic.