Charles Hayden sets off aboard the ill-fated HMS Themis with orders to destroy a French frigate and to gather intelligence from a royalist spy. On discovering French plans for an imminent invasion of England, Hayden must return to Portsmouth to give warning before it's too late. But the enemy have been lying in wait for him, and so begins a dangerous chase out into the Atlantic and into the clutches of a powerful French squadron…

Hayden and his men try in desperation to warn the Lords of the Admiralty. Failure will mean the invasion of England – and the guillotine for Hayden.