The boys complement each other neatly. Plum is the oldest; he's physically formidable, rather fearless, and quick to action if rather slow to thought. His scrappy younger brother Scram has the intelligence to plot schemes, both long and short, and the two brothers generally get along quite well. They are staying this summer with their cousin Richard, a reliable chap who, age-wise, falls in between Plum and Scram. Their respective parents are conveniently on holiday at the seaside, leaving a good fortnight that the boys have all to themselves. It does not take long for a mystery to present itself.
Reclusive neighbor Mr. Mocca has become a favourite subject of speculation. He has an impressive aquarium shed which houses exotic fish, a conger eel and an alligator. A turbaned Indian servant also arouses curiosity. On an evening visit to the shed, the boys discover the house in a strange state. Investigating, they find bloody fingerprints on a windowsill, black paint spilt haphazardly around a room, and an ominously sated alligator. Three sinister men nearly trap them inside the house during a late-night invasion, but the trio escapes, regroups, and quickly maps out a counterattack. Soon the boys are convinced they've stumbled upon a Nazi spy plot, and though the chief inspector has expressly told them to stay away from the men and the house, they consider it their youthful duty to bring the enemies to justice. Bullets fly, secret codes are intercepted and deciphered, false beards are employed, and through it all the young Britishers fight the good fight and work up quite an appetite.