Esme Colborne is about to marry Richard Trevannion. When she finds out she is adopted – from a working class family – she cannot allow Richard to marry so far beneath his station. Fleeing the life she knew, she starts working as a 'decoy woman', testing British undercover operatives who may otherwise reveal secrets in a moment of weakness. However, she still has feelings for Richard. Will she be brave enough to risk having her heart broken all over again?