"When Seraphin, Hans, and Sophie returned from their voyage to the moon, they didn't come home empty-handed--they brought with them aetherite, a miraculous substance that defies gravity! To keep their secret safe from the Prussians, the trio lives in hiding on a remote island in Brittany. Meanwhile, Seraphin's father has founded the International Society of Aether, a group of scientists dedicated to the peaceful pursuit of space exploration. But on the eve of their inaugural meeting, he vanishes without a trace! The Knights of Aether's last hope lies in a daring rescue of King Ludwig and Professor Dulac. They will find them on the farthest outskirts of the Prussian empire--the planet Mars!"--Provided by publisher.

Cover title. "Originally published in 2017 in French by Rue de Sèvres as Le château des étoiles, volume 3 : Les chevaliers de Mars"--Title page verso. Translated from the French.