Sarah's grandfather is accused of being a French spy. Suspecting that the Kembles are behind the accusations, Sarah begs Vincent to help. But Jack has other things to occupy his mind: Revenue man Marwood is after his blood.

Discovering that the Kembles are in league with a French spy - whose identity is known to him - Vincent sets out to prove König's innocence by facing the villains in their own stronghold.

A pair of sailors turn up from Jack's past accusing him of treachery, but are they telling the truth?