The Doctor makes a grand entrance at UNIT headquarters, with hugs for Rose Noble and Kate Lethbridge-Stewart. After greeting everyone he explains why he's here: this woman shows up everywhere he does. He shows slides, mostly of previous appearances in episodes, but some that have not been on-screen.

UNIT already knows about her. Her name is Susan Triad and she is releasing new software worldwide. S Triad Software.

The Doctor says S Triad = TARDIS, which everyone has figured out but they don't know why she's using it. Ruby realizes the woman's given name is Susan -- as in, the Doctor's granddaughter.

Ruby is confused. Wouldn't the Doctor recognize his granddaughter? The Doctor explains about regeneration. Ruby says she hopes he doesn't change his face. He says he's keeping this one a while.

They watch a little more and learn Melanie is undercover working with Susan and says she's nice.

THe Doctor says there's another mystery lady. He puts up photos of Ruby on the screen. Rose says Ruby was so cute in that photo. The Doctor explains about Ruby's birth mystery.

Morris wonders if Susan Twist and the birth mother could be the same person. Rose asks why he can't just take the TARDIS and see. The Doctor says that he was already there and can't cross his own timeline. Rose says but he never follows the rules. The Doctor says this area of time is clearly raw and open because the snow keeps following Ruby everywhere.

Harriet points out that this was 2004 and CCTV was a thing. Ruby has the tape at home but you can't see anything on it because of the snow. Morris says UNIT can sharpen it. Ibrahim takes Ruby to get the tape. Rose comes too, saying she never gets to do proper work and wants to go on an adventure.

Melanie tells Susan that the latest figures look great. Susan says she owes her success to her parents, who didn't have much but told her she could do anything.

Ruby and Rose go to Ruby's house. Carla gives her the tape, calling it the Legend of Ruby Sunday. When Rose mentions that her mum works for UNIT, Carla decides that she's going too. She insists that Ruby get Mrs. Flood to watch her mother.

Cherry warns Carla that the Doctor is trouble. Carla doesn't care.

They leave and Mrs. Flood refuses to get Cherry a cup of tea, warning her "he waits no longer".

At UNIT Kate asks the Doctor why he never mentioned a granddaughter or went to see her. The Doctor says he brings disaster and doesn't want to ruin her life. Kate thinks he brings joy.

Ruby and Rose return but security won't let Carla come up. The Doctor thinks that's ridiculous and changes the security protocols with his Sonic Screwdriver.

Meanwhile, Melanie scans Susan's DNA. It's human, as it is every day, but Kate explains catepillars don't know they are butterflies so she could change one day.

The Doctor asks Kate if she has a time window. She begrudgingly admits she does.

The Doctor takes everyone downstairs but says everyone stand back, all comms through him. He brings Ruby into an enclosed room. When she says she was born on Christmas Eve 2004, it begins snowing in the room even though the Time Window has not yet been activated.

The Doctor says it's time to bring them back to that day.

The Doctor and Ruby go back but there is a weird glitch and the woman who dropped Ruby off never shows her face. The Doctor tells the Colonel that is with them to check things out and he does but disappears, and a monster-looking thing shows up. The Colonel says he's in Hell.

The Time Window burns out and the Colonel's fossilized body is found, upsetting Kate, who orders a full analysis.

Melanie takes the Doctor to meet Susan Triad. Meanwhile, the team at UNIT analyzes the video and finds the monster on it. When they try reversing the video, they find there is a second TARDIS at the monster's heart -- how is this possible?

The Doctor tries to talk to Susan but it's time for her to go live so she refuses to speak to him but does acknowledge, sort of, that she has dreams of all of the other times she has appeared.

Kate calls the Doctor who tells her to bring Ruby to the Time Window. Kate tells Ruby not to ask questions, just cooperate. Rose and Ruby hug and Ruby goes into the Time Window room.

Meanwhile Susan starts her speech.

At UNIT, Harriet starts acting possessed.

The Doctor realizes the monster/Susan are Suketh. He tells Susan's staff not to touch her as she transforms. Suketh says to the Doctor that it is the god of death here to destroy the universe...