In today’s London, heartbroken and aristocratic detective Henry is settling into his new home when he gets called to investigate the theft of an original Jane Austen novel. Despite his preference to work alone, his pressuring boss sends the smart yet rather unempathetic literary expert Beth to his aid who happens to be an outspoken critic of the British class system. They meet at the scene of the crime, only to find out that the stolen book has been replaced with a perfect copy. Learning that the original is highly sought after by collectors, Henry starts to investigate without Beth.
While Beth laments to her sister Cassie and her artistically talented girlfriend Zoe about Henry’s inability to accept her help, Henry has dinner with his demanding yet loving parents who have a hard time accepting his decision to work for a living instead of settling with a wife and carry on the family’s legacy.
Shortly after Beth finds out that the theft wasn’t a one-off and informs Henry about it. Together they visit an auction expecting to find the culprit flogging off one of the stolen novels where they meet the American Anglophile Howard who happens to be a vivid collector of Jane Austen’s original prints. Howard and Beth hit it off, raising Henry’s suspicion that she is involved in the heists given her expertise in rare books. However, Henry agrees to accompany Beth to Howard’s Regency-style party at his home in the hope of finding out more. On the night of the event, Henry finds himself torn as he falls for Beth, while she increasingly becomes more suspicious of Henry’s background. Following an unsuccessful stake-out in Howard’s private library, Henry makes a move on Beth, but, much to her dismay, cops out at the last moment.
The following morning Beth and her flatmates google Henry and find out about his family. Beth realizes she has feelings for him despite his background, but is unsure whether he reciprocates them giving her difficulties in interpreting people’s manners. Henry and Beth meet to discuss the case but clash as Henry reveals his suspicions about her involvement in the thefts while she reproaches him for not being honest about his heritage. After telling Beth off for being prejudiced against aristocrats, Beth accuses him of being too proud to accept her help, and they part ways.
Henry decides to visit his parents at his loathed childhood home to clear his head when he learns that he was mistaken about Beth’s potential involvement. He finds a manuscript that once belonged to Jane Austen in his parent’s attic and decides to give it to Beth as an apology. At the same time, Beth has a heart-to-heart with Zoe about her inability to empathise with other people and learns that her flatmate has been struggling financially for a while. Henry arrives at Beth’s flat and gifts her the manuscript. Much to their surprise, it turns out to be Austen’s last chapter of her unfinished novel “Sanditon”. Given the value of this rare find, Henry comes up with a plan to set a trap for Howard to catch him red-handed and asks Beth for help, to which she agrees.
While watching over the public exhibit of the manuscript at Oxford, Beth tells Henry about her flatmate’s situation, leading him to conclude that Zoe must be the culprit as she had the means and the opportunity to steal Jane Austen’s books. As Beth is grasping the possibility that her friend is behind the thefts, Zoe sneaks into the exhibition and steals the manuscript right under their nose. However, Henry and Beth notice her in time and chase Zoe across town until her car falls into the river Themes. Beth finds Zoe and the manuscript submerged in the water and decides to rescue her at the cost of losing the valuable gift. Henry immediately dives for the manuscript to prove his love and trustworthiness to Beth, only to find it irrevocably destroyed. Despite being cross for stealing from her, the two friends make up as Beth learns to sympathize with Zoe’s difficult situation. As Henry and Beth bring back the original novels to the libraries, Henry comes to accept that love doesn’t go by the book and asks Beth to marry him.