Electra 'Ellie' McDonnell is a thief for British Intelligence.
She is not a spy, although what she does is the sort of things clandestine operatives are usually trained to do. She is skilled in opening locks - her Uncle Mick who raised her and is also a life-long thief even owns a legitimate locksmith shop where she officially works - and she can pilfer just about any trinket or bauble you desire and use her innocent looks to charm you while doing so. Working in the alleyways and in the shadows is second nature to her as is sitting patiently a long time somewhere out of view scoping the situation to know when it is safe to make a move. She would be very good spy.
That is what Major Ramsey of British Intelligence thought as well. He is the man who needed someone of her special abilities and inclinations and dropped just the right amount of hints about a wonderful little safe containing pricey little pieces of jewelry and enticed Uncle Mick to make a move. And had his strongarm boys ready to swoop in when the two came out of the building. This was especially galling to McDonnell because she KNEW there was nothing not right and warned her uncle about it but was overruled and suddenly both were in custody and a stint in prison was certain.
And McDonnell had had enough of prison. She might never have been incarcerated but she had been born in one to a now deceased mother. Mom was behind bars for reasons other than theft, though. Dad had been found stabbed in the heart with a butcher knife in December of 1915 and Mom was arrested and convicted of the crime and sentenced to hang. Her pleas of innocence were not believed but because they found she was a couple months pregnant, she was granted a stay. Then Electra was born and sent to live with Uncle Mick, Mom dying of the Spanish Flu soon after.
So thank you very much but no prison for Miss McDonnell in these scary days of 1940 when all curtains at night are drawn tight to darken the City from German bombers. Still Major Ramsey needs a good thief to open a safe and get some blueprints and that means he sets up Uncle Mick and Electra so that one of them can pull off the heist without being caught. His preference was the 24-year-old young woman.
And though she is not an agent, she finds the chance to do something for the war effort besides knit socks a terrifically exciting idea, not that she would let that handsome but annoying Ramsey know.