The Diplomat follows Laura Simmonds (Rundle) and her Barcelona Consul colleague and friend Alba Ortiz (Manteghi) as they fight to protect British nationals who find themselves in trouble in the Catalan city. Mixing the roles of lawyer, counsellor and cop, Laura and Alba's diplomatic skills are stretched to the limit by the stream of cases that walk through the Consul's doors.

The series opens with the unexplained death of a young British barman working on a yacht in Barcelona's notorious marina. While the Spanish police believe that the death was a tragic accident, Laura supports the boy's grieving father, Colin (Sapani) who is convinced something more sinister took place that night. As new evidence emerges and a murder enquiry is opened, Laura and Alba find links between the barman's death, organised crime and the British security services. Laura's quest for justice places her in real jeopardy, as she threatens to expose secrets that the British and Spanish will go to any lengths to keep hidden.