It's an extremely fan service-filled spin-off based around Illya being a magical girl, with a whole bunch of characters and references to the Fate series.
They exchange mana energy by kissing. It's fuckin' ridiculous.

It started off as a comedy series but then it got Fate real fucking fast. For the sake of comparison in part one it's mostly comedy with some good fights against "monsters of the week" sprinkled here and there. Come part 3 they have several direct antagonists who are very open about their desires to murder them combined with some pretty dark theming and imagery. (Think Heaven's Feel 2.0) It's the world of Fate/stay night being viewed through the lens of a young girl. Just because she's oblivious to the kind of stuff that goes on in it doesn't mean it isn't happening.