With the Septimus Heap series done, Sage has changed the main characters in the TodHunter Moon. That is, Alice TodHunter Moon. Spoilers ahead for Septimus. . .
Such as, the SFnal elements are introduced in more force after the reveal of the last Septimus book. Tod (as she wants to be called) on Midsummer Night learns that the secrete of the PathFinders; that they were once interstellar travelers, and some of them still have heriditary gills that let them breath in liquid (acceleration issues). The downside of that is that to find out if you have gills, you have to breath in water. If you don't, you drown. There's no turning of the magic into science, though
But Tod's aunt was spying on their secret meeting. And the next day, her father vanishes. Others do, too. Tod's friend Oskar figures out she's next, and stop it, and his brother finds out from a fisherwoman a place where his sister Ferdie might have been taken.
The youngsters set out. Discovering perils and adventures. And returning to find something worse.
It involves the magical properties of lapis lazuli, a wizard whom Tod met when she was four, a Shakespearean sonnet, a haughty princess from a snow-bound land, Tod doing something earlier than expected, Simon and Lucy's little boy, cages, and much more.