The summer term ends in a blaze of glory for Frank Mannix, school hero and cricket star, who sets off for what he expects to be a grand holiday of salmon fishing and grouse shooting in the west of Ireland with his uncle Sir Lucius Lentaigne. Instead, his journey begins with an unceremonious clash with an abrasive elderly gentleman on the steamship gangway, resulting in a sprained ankle and a broken fishing rod. Arriving at Rosnacree House, Frank meets his cousin Priscilla, a fifteen-year-old girl of relentless energy, determination, and skill in rebellion, who immediately recruits him for far more important matters than fishing. Priscilla seizes upon the notion that a mysterious stranger staying in a nearby cottage and renting an old boat is a dangerous German spy mapping out the bay for an invasion.
Although Frank is initially skeptical, his youthful imagination is soon inflamed, and he joins Priscilla in a series of sailing expeditions across the complex, tide-swept maze of the bay's three hundred and sixty-five islands. Their pursuit leads them into encounters with local characters, including the laconic boatman Peter Walsh, young Jimmy Kinsella, and a cheerful lady named Miss Rutherford who claims to be collecting sponges and zoophytes for the British Museum. As the cousins sail the unpredictable waters in Sir Lucius's racing boat the Tortoise, they find themselves repeatedly thwarted by the shifting tides, grounded on mudbanks, and sidetracked by local legends about rats and fever on the mysterious island of Inishbawn.
Meanwhile, high politics and family drama intrude upon Rosnacree House when Lord Torrington, the Secretary of State for War and the very man who shoved Frank on the gangway, arrives unexpectedly in search of his runaway daughter, Lady Isabel, who has seemingly vanished into the western landscape. The narrative weaves together the children's imaginative manhunt with the polite tensions of the adults, as Priscilla and Frank chase false leads from island to island, gradually drawing closer to the truth behind the mysterious strangers and the unfolding family crisis. Through sparkling prose and vivid descriptions of the Irish coast, the story builds toward a gentle resolution where youthful playfulness meets the absurd realities of the adult world.
