The Saskia books are set in the Saskia universe which exists just an infinitesimal distance from our own, in a direction impossible to describe. Everything in it is identical to our own - except that it's possible for two girls to have the same super powers as the comic book Supergirl. Not only that, time travel is also a reality, in a limited sense at least - limited, that is, to the same two girls, usually at any rate.
The affairs of these two unusual girls are watched over by a disembodied being they come to call a Guardian of Reality. In return for giving them the powers they enjoy, it requires them to undertake tasks it cannot do itself - like saving the world and meddling with events in the past.
The two girls try to act like two normal teenagers, sometimes it's easy, sometimes it's hard. It's made many times more complicated by the fact that one of them isn't a teenager - or even a girl.