Set just before and during the Great War, the novel recounts the adventures of Martin Hillyard, a secret service agent, and his army officer friend Harry Luttrell who aims to restore the battered reputation of his regiment.

After University Hillyard worked as a playwright while Luttrell felt it his duty to take a commission in his family's regiment, the Clayfords, to redeem its reputation after disgrace in the South African Wars.

Most of the house guests attend the racing at Goodwood, but Lady Splay's orphaned niece, the 18 year old Joan Whitworth, affects to reject conventional society and declines to go.

With assistance from an influential Spanish tobacco smuggler, José Medina, Hillyard obtains details of German submarine movements and discovers how enemy messages are being passed between Berlin and Madrid.

At breakfast the next morning, the party are astonished by a London newspaper report of an incident said to have taken place in the house the night before: the tragic death by chloroform of Mrs Stella Croyle.

Hillyard learns that during the night – but well before the death actually occurred – a chauffeur had arrived at the newspaper's offices bearing a letter under the signature of Sir Chichester, but (unknown to the editor) in Stella's handwriting.

[7] Mason's secret service work during the war provides the basis for Hillyard's activities, including ostensibly holidaying in the Mediterranean on a steam yacht, and calling in – apparently innocently – at various ports of interest in Southern Spain and the Balearics.

[10] The Aberdeen Daily Journal considered that the author had used the fruit of his experiences in the war as a secret service man to "capital effect" in this novel, praising him for "his old mastery of a complicated plot, in which mystery, romance, and picturesque settings compete with a merry wit for predominance in a story of colourful interest".

"[12] Mason's biographer Roger Lancelyn Green, writing in 1952, considered The Summons a first class novel, though flawed due to the welding together of two disparate plot threads: the secret service exploits of Martin Hillyard, and the tragedy of Stella Croyle.