The Company of Strangers — Robert Wilson

The stifling summer streets of Lisbon are teeming with spies and informers when Andrea Aspinall, an English mathematician turned spy, disappears under a new identity. Military attach? Karl Voss, experienced in the illusions of intrigue, arrives in Lisbon under the German Legation, though he is secretly working against the Nazis so that atomic and rocket technology do not find their way into Hitler's hands.
In the lethal tranquility of a corrupted paradise Andrea and Karl meet and attempt to find love. Tragically, a night of violence leaves Andrea the keeper of a secret that triggers a lifelong addiction to the clandestine world. From Portugal to England and finally Cold War Berlin, she gradually discovers that the deepest secrets aren't held by governments, but by those closest to you.