When Emilie and Daniel arrive in Silk Harbor, Professor Abindon, an old colleague of the Marlendes, warns them that she's observed something strange and potentially deadly in the sky, a disruption in an upper air aether current. But as the Marlendes investigate further, they realize it's a ship from another aetheric plane. It may be just a friendly explorer, or something far more sinister, but they will have to take an airship into the dangerous air currents to find out.

Emilie joins the expedition and finds herself deep in personal entanglements, with an angry uncle, an interfering brother, and an estranged mother to worry about--not to mention a lost family of explorers, the strange landscapes of the upper air, and the deadly menace that inhabits the sky world.