Nissa, Madoc, their siblings and the Duke’s son Kerr travel north to Solwintor to seek the source of Madoc’s two books, written in a strange language. At a factory west of the town of Grenska they learn about a catastrophe a thousand years earlier caused by the formation of their world’s second moon, and about the Stronghold, a cavern where people from many countries are attempting to recreate the lost crystal-based technology from before the catastrophe. Nissa, dismayed that she’s assigned to the sewing room at the Stronghold, uses her assignment to instill a sense of experimentation among her new friends. More crystals are needed to power the devices recreated at the Stronghold so an expedition is sent to Dulno Lake in an attempt to retrieve crystals from its depths. Nissa accompanies those sent, including her companions. She helps the expedition fight against perils, including those who oppose them and monstrous fish at the bottom of the lake. Will the expedition be successful, and will Madoc ever find the source of his books?