The beloved Crawleys and their intrepid staff prepare for the most important moment of their lives. A royal visit from the King and Queen of England will unleash scandal, romance and intrigue that will leave the future of Downton hanging in the balance.

Henry Fallon, who is wheelchair bound and is suffering from a neurological disorder, apparently commits suicide and Morse has mixed feelings working on this case as he was once engaged to the dead man's wife, Susan Fallon. The body was found by his son-in-law Peter Rhodes, a local antiques dealer and the victim's wife was in London. When Fallon's doctor advises Morse that Henry didn't have the physical capability to hold a gun to his head, the police realize they may have a murder on their hands. When the police are told that the business arrangement between Fallon and Rhodes had fallen apart, they also have a suspect. Lewis is concerned that Morse is too close to Mrs. Fallon to be objective but in the end, the perpetrators are identified.

Eve is a bored, whip-smart, pay-grade MI5 security officer whose desk-bound job doesn't fulfill her fantasies of being a spy. Villanelle is a mercurial, talented killer who clings to the luxuries her violent job affords her. Everything changes when they cross paths.