When Euphemia Martins accompanies her employer Richenda Muller to London, she envisages a pleasant few days spent taking tea and visiting the shops. Her hopes are dashed when Richenda gets her involved in a suffragettes' march which soon turns nasty. Euphemia finds herself getting quite physical in the defence of her fellow marchers when the police turn on them, and she is imprisoned for her pains. Euphemia soon realises this is no ordinary imprisonment when her old acquaintance Fitzroy enrols her in the latest of his schemes for King and Country. An important civil servant has been murdered, and Fitzroy believes one of the women imprisoned with Euphemia knows the reason why. Euphemia must work out who it is... a task made more complicated when one of her fellow prisoners is also murdered. It's up to Euphemia, with help from Richenda and Richenda's brother Bertram, to trap the guilty party - and fast!