The thing is. Well, it goes like this:
The Doctor Internalizes the whole 'I'm Dangerous To Other People, my lifestyle gets people hurt' and self isolates for v long while
Missy2 Happens. She straight up just kind of makes the offer Automatically, and, like, surely it's impossible to mess this up any further, right. Like. She has to stay away from Humane Connection because they don't deserve to get hurt by her but he Doesn't Count in that calculation, right. not just cuz he's Not An Innocent but also because he wouldn't let himself get hurt, regardless of what he Claims there's something in his nature that will make him duck out on here before shit gets too real, right- YES, it is integral to my view of things that the doctor Never Ever finds out what Actually happened with Missy(1) in the end. And never gets entirely over the perceived betrayal. and Missy2 straight up becomes aware of this at some point and takes 200 million psychic damage And Does Not Bother Correcting Her. Conflict and hatred on planet all of them. ANYWAYS THIS ISNT THE POINT.
during some Hijinks in medieval coventry this teen girl ends up rly bonding with the Doctor, not only fascinated by her confidence and the respect the Doctor treats her with but also the knowledge and (scientific) work methods. Agnes- that's the girl's name- directly asks the Doctor to be allowed to join her as a student/apprentice and the Doctor is like Absolutely Not due to said strong strong belief in the risks to companions.
Agnes sneaks aboard stoaway style . Missy2 fully notices and says nothing cuz he's a little shit. And he's not being paid enough to care. And he thinks it'll do eighteen some good. But mostly he's being a little shit and sticking to the letter of some argument they had about interfering in her Human Interactions tm rather than the spirit.
Eighteen finds out Agnes is there at the worst possible timing- you know those days you're Immediately surrounded by drawn guns the moment you step out of your tardis door?- and is very angry and neurotic about the whole Endangered Child On Her Conscience Thing.
"We are getting you back home as SOON as possible. Do you... know what Year it was." "Oh . Um. (t minus Like Five Minutes From A Timelord's Eye View until the black plague hits england)".
Eighteenth Doctor blue-screen-of-death as she tries to Not freak out / break down about this. The most clenched anyone's jaw has ever been.