German-Brazilian Isabel Neumann delights in creating seashell art, but it's her mathematical ability that lands her a job at the American air base in Natal, northern Brazil during World War II. She doesn't need a calculator to determine the correct weights and balances for the Air Transport Command's cargo planes.

Daniel Lambert, an American transport pilot based at Natal, endures the taunts of combat pilots that he is "allergic to combat." His flying skills win him respect, however, and his friendship with Isabel deepens, even as a new source of trouble looms.

Isabel is caught in the crosshairs of a German saboteur who is obsessed with her. He insists that she belongs with him, and demands that she help him sabotage the Allied base. Her growing relationship with Daniel angers the Nazi, who will do anything to get rid of him. What will happen to Isabel if the madman captures her?