Filming began in July 2015.[12] Cornwall was used for many of the harbour and beach scenes, including Holywell Bay, Kynance Cove, and Mullion Cove.[13] Harefield House in Hillingdon, outside London, served as the location for the island mansion.[14] Production designer Sophie Beccher decorated the house in the style of 1930s designers like Syrie Maugham and Elsie de Wolfe.[15] The below stairs and kitchen scenes were shot at Wrotham Park in Hertfordshire.[16] Railway scenes were filmed at the South Devon Railway between Totnes and Buckfastleigh.[17]