Since she was orphaned, Sue Trinder (Sally Hawkins) has been brought up amongst pickpockets and charlatans. She has been protected and cared for by Mrs Sucksby (Imelda Staunton) who believes Sue is too good a girl to get directly involved in such a world. But when Mrs Sucksby's old friend Richard Rivers (Rupert Evans) offers 20-year-old Sue £3,000 to assist him in one of his scams, Mrs Sucksby cannot resist. Passing himself off as a proper gent, Rivers has befriended a young lady, Maud Lilly (Elaine Cassidy), who stands to inherit a fortune when she marries. However, Maud's maid has recently left her service and, without a chaperone, Rivers' access to Maud is limited. He wants Sue to be accepted as the new maid and to help him win Maud over. Once married, Rivers plans to have Maud committed to an asylum and he will then take her fortune for himself. Arriving at Briar, the estate where Maud lives with her wealthy bookish Uncle Lilly (Charles Dance), Sue enters another world. Maud and Sue are of a similar age and appearance but their experience of life couldn't be further apart. Maud's existence is one of wealth and prosperity inside a grand house, where her duties are limited to nightly readings to her uncle and his friends. Over the course of a few months, Sue and Maud become friends. But Sue's growing feelings for Maud cannot prevent her from going through with Rivers' scam. On Sue's advice, Maud accepts Rivers' proposal of marriage. Will this event lead to Maud's demise and Rivers' great fortune? Now married to Maud, Rivers' next plan is to get her admitted to the asylum. But in a dramatic twist it is Sue that Rivers has certified. Suddenly Sue realises that Maud was in on the plan from the very beginning.

Having disposed of Sue, Rivers takes Maud to London to Mrs Sucksby's house. There Maud learns that Mrs Sucksby is the mastermind behind her escape and Sue's downfall. Maud is plagued by doubts over her treatment of Sue and the fake life that she presented. In reality Maud was orphaned and brought up by her Uncle, a cruel man, who has abused his niece and hardened her heart. The nightly readings in his library were not as innocent as it appeared to Sue. Maud was forced to read pornography to her uncle and his friends. Tormented by the cruelty of her existence, Maud found in Rivers a way out. By marrying him, she could escape the prison of her uncle's house. But what Maud hadn't expected was to be so affected by her friendship with Sue - or indeed that she would fall in love with her. In order to save herself from her uncle, Maud had to reconcile herself to hurting Sue. But even Maud couldn't have imagined that her world would be further turned upside down. Mrs Sucksby reveals that through a series of twists and turns Maud and Sue were switched at birth — they have been living each other's lives. Can Maud continue to live with this newfound knowledge, knowing that Sue has been deserted by all those she once loved? Mrs Sucksby explains to Maud that she has no choice but to comply with the rest of the plan. Maud is further devastated to learn that they have no intention of rescuing Sue from the asylum.

Acknowledging the wrong she has done, Maud knows she must escape Mrs Sucksby if she is to rescue Sue. She manages to get out and finds her way to an old business associate of her uncle's, but he refuses to help and sends her away. Alone in the alleyways and streets of London and unaware of the dangers that lurk, Maud falls prey to sexual predators and realises she has no choice but to return to Lant Street. Mrs Sucksby is overjoyed to see her and Maud discovers that she is in fact Mrs Sucksby's own daughter. Meanwhile, employing all the underhanded tricks that her childhood has taught her, Sue escapes from the asylum. She heads for London where she plans to get her revenge on Maud for what she sees as betrayal. At Mrs Sucksby's, Sue confronts Maud, Mrs Sucksby and Rivers; a huge row ensues and in a struggle Rivers is killed. Despite Maud being guilty of Rivers' murder, Mrs Sucksby confesses to protect both girls and soon after is executed for the crime. Maud disappears and Sue, realising that Maud was just an innocent in Mrs Sucksby's scheme, sets off to find her. Acknowledging their feelings for one another, the two women are reunited.