Throughout Season 4, the show explores themes of love, family, and social class. Emily and the Duke's relationship continues to be at the forefront, as they navigate the challenges of their different backgrounds and the scrutiny that comes with their love.

The season opens with Emily returning to London, determined to put her detective skills to use and make a name for herself in the male-dominated world of crime solving. However, she soon finds herself entangled in a new mystery when a wealthy merchant's wife goes missing.

The season also delves deeper into the supporting characters, including Emily's cousin, Hattie, who finds herself entangled in a romantic affair with a charming but untrustworthy suitor. Meanwhile, the Duke's family continues to play a significant role, particularly his mother, who will stop at nothing to ensure her son's reputation remains intact.