Spring, 1941. France has fallen but the Free French naval forces are in no mood to surrender. Royal Navy Sub-Lieutenant Harry Gilmour is also ready for action, despite the horrors of his first taste of submarine warfare.
When he is appointed as British Navy Liaison Officer aboard the Free French submarine Radegonde, he finds it anarchic, disorientating - and very French. Within its claustrophobic confines, suspicion and misunderstanding are rife.
So when Radegonde is sent on a mission to Martinique, it's vital that these proud men learn to work together, especially as it seems everyone from Churchill to de Gaulle - not to mention Hitler - has a stake in the outcome.
Will Harry be able to navigate these dangerous waters safely and return with hard-won wisdom, or will old enemies arise to sink him?