Every Sunday Gwen and Jill make a sandcastle for the weekly contest, but every Sunday some fink wrecks it when they are gone for lunch. And so the two young sleuths who previously flushed out Something Queer at the Ball Park (1975) and . . . the Library (1977) uncover clues--a spot of salami, a flipper print--but overlook the most telling one, little sister Nan's unhappiness at not being included in the castlemaking. Then at the 4th-of-July final contest a flippered Nan is apprehended in the act. Gwen and Jill save their castle and win the prize, and as punishment Nan is not allowed to watch the fireworks. You'll recognize the series' usual mix of trifling mystery, peppery pictures, and a pinch of spite.