The fifth Septimus Heap book. While the plot is stand-alone, it's tied more closely to Queste than the earlier books were to each other. (Spoilers for the earlier books ahead.) Interestingly enough, there are some clues larded in here that point to its actually being a far future story.
In fact Queste is the "there" and this is the "back again." To be sure, Septimus is already back again, and Marcia informs him that now that he completed the Queste, he is a Senior Apprentice -- rather early, but then, he completed the queste. He takes advantage of his new freedom to go as he wishes to set back on Spit Fyre to rescue Nicko, Snorri, Jenna and Beetle. He finds them with Jenna's father Milo in the harbor, and disclined to be rescued, until Milo's flamboyant style sets off Jenna. They set out on Spit Fyre, but there's a storm brewing.
Meanwhile, Aunt Zelda has decided to alter the tradition of the Keepers: she wants Wolf Boy as her apprentice, and nothing actually requires the Keeper to be a woman. Which means that Wolf Boy is sent to the Port Witch Coven, to feed the Grim. He overhears Simon Heap worrying about Lucy, who's vanished, and when he actually goes to the Coven, he finds they are holding her prisoner. Their eventual escape sees them leaping on to a ship that's leaving port.
And it leads to where a lighthouse is tampered with, causing trouble for Milo and his ship, not to mention Nicko and Snorri.
It all ties together involving a cat, another Extraordinary Apprentice, a tower, an Ice Tunnel far from the castle, an enormous number of genies, and rats leaving a ship, twice.