In an effort to recover the Swedish scientist Dr. Hansen, who's been missing for six months and is unwittingly giving his work to the Germans, British agent David Grant catches the night boat to Dublin. All of Hansen’s recent atomic research work is being sent to a German agent, Keitel, in neutral Ireland. Robert Wilson, a British secret service agent, also travels to Dublin and retrieves the secret papers from Keitel, but on the return journey, he's murdered by a Nazi agent. Grant becomes suspicious of a man who used a phoney name in a Dublin hotel, Paul Faber, and along with fellow agent Tony Hunter, heads to London to investigate. Grant takes a job with Faber’s consultancy firm, and ingratiates himself to the Nazi spy by pretending to be a shady character and deserter from the armed forces. In return for his silence, Faber demands Grant agree to a marriage of convenience to Marion Decker, an Austrian immigrant. Faber comes to suspect Grant is an undercover agent and accompanies him to Devon where Hansen is being held in a former monastery owned by outwardly respectable Sidney Vane.