A Quintarian Science Guild survey ship makes a terrifying discovery on Thaumatar, home planet of the original World Door builders. Thunder sees a mysterious flickering white mouse on the Island of Blue Monks while Bartholomew and his trusted companions Edmund the Rabbiton and Oliver travel to the lost city of Cathne in search of Madam Beffy's missing father. With help from Captain Mudgeon of the Quintarian Naval Armada and a six foot tall alien grasshopper name Hoppy, Clara, Thunder, and Lightning must find the six silver medallions hidden beneath shadowy Tenebra Military Prison on Betshannk Island. Together, Bartholomew and his entire crew of stalwart adventuring companions must solve the riddle of the impossibly powerful and deadly Black Sphere before it brings an end to their universe.