Alice, Alex

A year after his mother disappeared in her hot air balloon, Seraphin and his father receive a clue that sends them to a Bavarian castle where lurks a force that would stop at nothing to conquer the stars.

Seraphin, Hans, and Sophie travel to Mars in search of Seraphin's father but realize they were tricked by Gudden. When Seraphin gets separated from the group he must rely on a shape-shifting Martian to see him to safety. When he arrives at the Martian's home, he sees Gudden has duped them into thinking he's a god.

"When Seraphin, Hans, and Sophie returned from their voyage to the moon, they didn't come home empty-handed--theybrought 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!"--Amazon.

"A year after his mother disappeared in her hot air balloon, Seraphin and his father receive a clue that sends them to a Bavarian castle where lurks a force that would stop at nothing to conquer the stars"--OCLC.