At the beginning of the episode we see Clara trying to run her dual lives. One of dating Danny and Teaching English, and one adventuring with The Doctor. She's running low on fuel. Suddenly the Doctor shows up at Coal Hill as the new
Caretaker, John Smith. For those not in the know, a Caretaker is the British version of Janitor, or Custodian or general all-round fix-it person in a school, like a building Super.
The Doctor shows his ongoing dislike for soldiers when introduced to former Sergeant, Danny Pink by another Coal Hill teacher. Constantly referring to him as a P.E. teacher, as if thats the only thing an ex-member of the military could do in a a school. Certainly not teach math (or as they call it Maths)
The Doctor sees that the person that she's been dating must be at the school, and when she walks off to talk Shakespeare's the Tempest, he thinks Adrian is the one, and he approves. Meanwhile The Doctor is planting devices around the school, he's looking for a deadly robotic alien soldier in the vicinity, that has already killed an investigating police officer. The Doctor is interrupted by a curious student and Clara. He reveals his new invention to Clara, an invisibility watch.
After Clara and Danny leave, the Doctor's arrogance to show off surfaces as the curious student shows up again and he shows her the TARDIS. Continuing to work on Skovox coming back, the Doctor creates what looks like a proton pack out of Ghostbusters, and in the middle of parents night the creature returns. Danny witnesses Clara running from the alien as a decoy and intervenes to buy the Doctor just enough time to shut it down. Danny saving Clara and the Doctor saving the world, calls into question who is the Caretaker?
The final scene gave us a bit more on Missy. And I won't say anymore and to avoid spoilers.
Next time on Doctor Who: Kill the Moon