Canadian Tom Merritt has discovered a substance, Compound X, that will cure infantile paralysis, which he extracts from a mine at Caribou, Canada. A country at war with Canada (unnamed because while Canada and the British Empire were at war with Germany, the USA "officially" wasn't in 1940) discovers that Compound X also contains magnetic properties which will make their mines effective against the British fleet, and dispatch one of their intelligence officers, John Kettler, to work undercover to ensure a steady stream of Compound X for the Fatherland.
