It's a bad day at Coal Hill School. Clara discovers that the Doctor is acting as the temporary caretaker in an effort to protect the planet from an intergalactic killing machine who is stuck on earth. Not only must she deal with the Doctor's shenanigans, she has to introduce him to Danny Pink.
Gareth Roberts has had offered some fun in recent years by putting the Doctor on Earth in ordinary-seeming settings where things just keep getting funnier and funnier. He's played this a couple of times with the inimitable James Corden as his put-upon henchman. In this one he throws a spanner into Clara's budding romance with Danny.
The first half plays nicely with the Almighty Janitor trope. The Almighty Janitor is the guy without whom the place cannot operate, who has access to all things. It's a fairly well-known story-telling technique. The tvtropes web site has a long section on the Almighty Janitor, with examples ... many of them from the real world.
It's an episode that is well integrated with the season arc. The only cavil I have is that I can't follow the multiplicity of regional English accents. Is it possible that the series will eventually come with subtitles?