On the Double

British intelligence identifies a capable lookalike for General Sir Lawrence MacKenzie-Smith. They select American Pvt. Ernie Williams, a talented mimic with a striking resemblance. The plan hinges on Ernie stepping into the General's role to mislead German intelligence.

MacKenzie-Smith, after initial reservations, agrees to the impersonation plan. Ernie is officially introduced as the proposed double. However, a vital detail is withheld from him: that he will serve as a decoy target for German intelligence.

Lady Margaret returns from Canada earlier than expected, seeking a divorce due to neglect and womanizing. She immediately recognizes Ernie as an impostor when she sees him. She is briefly brought into the operation and begins to understand the stakes of the deception.

Margaret shares intimate details about MacKenzie-Smith to help Ernie maintain his disguise. The two begin to develop sympathy for each other as they spend time together. Their relationship gradually grows into romance.

Ernie grows weary of the dangerous deception and attempts to resign from the project. He is reminded of the high stakes and pressures around him, but the operation continues under higher command.

A plane carrying MacKenzie-Smith is shot down, killing all aboard and forcing Ernie to continue the deception. With the real general dead, there is no easy way to reveal the truth without risking Allied plans. Ernie is now trapped in a permanent impersonation.

After a wild regimental party, Ernie is kidnapped by one of MacKenzie-Smith's mistresses and a traitorous officer, both Nazi agents. He is taken away from England to Berlin under guard. The abduction marks a turning point as the deception travels behind enemy lines.

Ernie is flown to Berlin and interrogated about Allied invasion plans. He cannot convince the Nazis that he is a decoy and ends up divulging non-existent invasion plots. The interrogation intensifies the stakes of his cover.

Left alone by his captors, Ernie secures a list of German agents in Britain and makes a daring escape from German Headquarters. The information on the list gives him a glimmer of leverage against the infiltrators. He begins a perilous journey back to Allied lines.

A sequence of comic errors unfolds in a Berlin cabaret as Ernie crafts a disguise. He disguises himself as a Luftwaffe pilot and boards a bomber bound for England. The risky ruse culminates in his parachuting out over Britain.

Ernie is captured after landing and brought before General Wiffingham, a friend of MacKenzie-Smith who is secretly the chief Nazi infiltrator. Reading the list on his person, Ernie realizes the depth of the betrayal. The confrontation sets up the dramatic showdown.

Ernie is rescued before Wiffingham can order his execution by firing squad. The invasion proceeds as planned and Margaret and Ernie are reunited in London, where they confess their love for one another.