Maggie Flynn isn't your typical 1960s mum. She's a spy, an unsuspecting operative for MI5, stalking London's streets in myriad disguises.

Widowed and balancing her clandestine career with raising a Beatles-mad teenage daughter, Maggie finds comfort and purpose in her profession – providing a connection to her late husband, whose own covert past only surfaced after his death.

But Maggie's world spins out of control when a chance encounter with a mysterious Russian agent triggers a chilling revelation: he knew her husband. And what's worse, the agent suspects someone on home soil betrayed him.

As Maggie searches for answers, she'll question everyone – and everything – she thought she could trust. In the murky and perilous world of espionage, can she outsmart those determined to keep her silenced?