Tom Johnson was one of those extraordinary characters that history throws up in times of crisis. Born in 1772 to Irish parents, he made the most of the opportunities that presented themselves and was earning his own living as a smuggler by the age of 12. At least twice, he made incredible escapes from prison. When the Napoleonic Wars broke out, his well-deserved reputation for extreme daring saw him hired–despite his by then extensive criminal record–to pilot a pair of covert British naval expeditions.

But Johnson also has a stranger claim to fame, one that has gone unmentioned in all but the most obscure of histories. In 1820–or so he claimed–he was offered the sum of £40,000 [equivalent to $3 million now] to rescue the emperor Napoleon from bleak exile on the island of St. Helena. This escape was to be effected in an incredible way–down a sheer cliff, using a bosun’s chair, to a pair of primitive submarines waiting off shore. Johnson had to design the submarines himself, since his plot was hatched decades before the invention of the first practical underwater craft....