Sixth book in the Septimus Heap series. Spoilers for the first five ahead.
Marcia goes to Banish Tertius Fume -- and good riddance, no doubt, except that Fume manages to get Alther Banished with him. Which does not, at first, touch off many changes.
But Lucy Gringe comes back to the Castle, bearing letters, because she and Simon want to reconcile with their families. Jenna doesn't read her letter, Sarah makes a fuss over Lucy -- and meanwhile, a Thing kidnaps Simon.
One reason why is that Jenna is worried about something in the castle. She's sure it's Darke. And Septimus gives her the brush off because he's too worried about his Darke Week, because he's going to try to rescue Alther from the Banishment. Marcia is worried about it too, but when she brings in Marcelleus, he backs up the time that Septimus has choosen.
Beetle agrees to come with Jenna, and they discover a Darke Domain, which is serious trouble. Merrin Meredith still has not forgiven Septimus for turning out to be the real Septimus.
And the story winds on from there, involving the spread of the domain, a duck behaving foolishly, four adolscent rats, a witch's cloak, two dragons, Beetle having to fill in for Septimus more than once and not happy about being second choice, a whirlpool, some calculations, escaping out the window, and things that are reversed.