In a time and place which might be now, people with gills, outcast larkers, live in secret communities. They have houseboats along the river. Concealing their gills from land lubbers, they scour the mudbanks, trade their finds and live off their wits. Thirteen-year-old Effra has been supporting her brother, Fleet, alone since their beloved grandfather died six months ago. When merciless Rivermun, a larker gone bad, threatens Fleet, Effra's quests begins. Rivermun asks for the impossible - he wants to overpower Mother River, to possess the river serpent's pearl and for age-old debts to be settled. Effra must bargain with the imposing Mother River, dive into the underwater parts of the city, venture deep into the Rat Queen's lair and confront the terrible river serpent to save not only Fleet, but everything the Larkers stand for.