Hannay had previously appeared in The Thirty Nine Steps (1915), his most famous adventure in which he battles German spies across England and Scotland, and two books about his activities during the First World War, Greenmantle (1916) and Mr Standfast (1919).
It is some time after the war, and Sir Richard Hannay is living in rural tranquility, having bought Fosse Manor and married Mary Lamington (both featured in Mr Standfast); they have a small son, named Peter John. Hannay's new friend, local doctor Tom Greenslade, a well-travelled and learned man, talks ominously one night of psychology, the subconscious, thrillers and post-war society. Later, Dick reads a letter from his old boss Sir Walter Bullivant, warning him that he will soon be asked to undertake another job for the country.
Next day, Julius Victor visits Hannay and tells him his daughter Adela has been kidnapped and held hostage, asking Hannay to help find her. Later that day, MacGillivray visits and tells Hannay of a sinister criminal organisation, controlling the mass of disturbed and disordered minds left over from the Great War, and tracked by the police forces of the world. Faced with capture, the leaders had taken three hostages, Victor's daughter, an aristocratic student and a young boy, sending each of their families a mysterious poem to prove the kidnappings were linked. He also explains that they have only until June to round up the gang, and that the hostages must be safe by then.
Hannay is adamant that he cannot help, but his third visitor that day, Sir Arthur Warcliff, tells him about his missing son, and Hannay is drawn into the chase. That night, he lies awake pondering the lines of doggerel sent to the families, and connects them to something Greenslade had said recently. Next day he tells Greenslade all, and bids him remember where he drew his phrases, two of which, concerning a blind woman spinning and a barn in Norway, matched verses from the poem, while the third in Greenslade's speech referred to a curiosity shop run by an elderly Jew, which seems to bear no correspondence to the poem's reference to the "Fields of Eden". Greenslade is baffled, but Hannay recalls a hymn mentioning the Fields of Eden, which Greenslade connects with his vague memories.
Another day's pondering gets them no closer, until Hannay breaks his pipe. Suddenly Greenslade remembers an evening in a country pub, where a man named Medina had broken his pipe as he hummed the tune, and at the same meeting mentioned the ideas echoed in the poem.
Hannay heads to London to meet Macgillivray, and briefly runs into his old friend Sandy Arbuthnot. Macgillivray briefs him on their enemies, and soon after Hannay meets with Medina, a handsome and accomplished man who Hannay finds he likes a lot, but doesn't yet take into his confidence. He sees Sandy again, who is suspicious of Medina, and the three attend a meeting of an elite dining club, where something Medina says affects Sandy extremely - he becomes rude and angry, and tries to drag Hannay away, but Hannay refuses and walks home with Medina. They stop at Medina's house for a pipe, and there Hannay has a strange dreamlike experience of which he remembers little, only later realising that Medina attempted to hypnotise him and that he somehow resisted.
He wakes next day feeling ill, and visits a Doctor Newhover, whose name was planted in his head and who refers him on to a masseuse named Madame Breda, in whose house he also sees a strange young girl. He is again hypnotised, by unseen hands and a strange voice, and again resists. He later reports his experiences to Sandy, who urges him to watch Medina closely and makes plans to investigate the house of the masseuse, and then to take his researches to Europe.
For some time Hannay hangs around Medina, one day attending a secret dance-hall with his friend Archie Roylance, where he sees a beautiful girl with dead eyes led away by Medina's suspicious butler, but learns little. He visits Newhover again, and learns that he plans to head to Norway. At last, visiting Medina, he is taken to the library where he was first hypnotized, and introduced to Medina's mother, a striking, frightening, blind old woman. He is again hypnotised, and made to do demeaning tasks, until they are sure he is under their control. He hears of Medina's plans to meet with one Kharama, and learns the gang plans to break up by midsummer, before he faints from exhaustion.
Hannay arranges with his friend Archie Roylance to be flown home from Norway when the time comes, and is taken to meet Kharama, an impressive but sinister Indian who discusses hypnotism with Medina. Later, he gets gets a note from Sandy, arranging a meeting. Telling Medina he is ill and needs a week's rest, he fixes a rendezvous with Roylance and heads home to Fosse, where he sets up a pretence of being in his sickbed. He meets Sandy and they share what they have found, and then slips onto the boat taking Dr Newhover to Norway.
Arriving there, he sees Newhover head off by boat, and follows at a discrete distance. Knowing Newhover's heading, he leaves his boat at the village before, bidding the pilot to await his return, and heads overland to Merdal. There, not wishing to be seen at the inn by Newhover, he approaches some locals to find lodging, and is amazed to meet Herr Gaudian, a German engineer he met during the events of Greenmantle. Gaudian reveals he knows Newhover, in truth a former German agent, and agrees to help Hannay. Hannay takes lodgings alongside Gaudian, and learns of a farm in the hills leased to an Englishman; next day the watch Newhover travel up there, and the man he replaces head down to the village.
They spy out the place, but learn little until the second night, when Hannay, heading to the farm, sees Newhover hurrying anxiously down the hill; he heads into the scrub to avoid being seen, and comes across someone scrambling through the bushes. The man falls into a stream, and Hannay rescues him, and recognises Lord Mercot, one of the hostages. They feed and bathe him, and hear his broken story of hypnotised abduction, but then must persuade him to return to his captors, until such time as the other hostages can be found, promising Gaudian will be keeping an eye. The brave boy heads back to his prison, and Hannay heads for his rendezvous with Archie Roylance. Despite the weather, he makes it safe back to England, and returns to London and Medina.
He spends some days with Medina, including another meeting with Kharama, to little avail, but on hearing the hymn mentioned by Greenslade at the start of his quest he is inspired to investigate London for a place suggesting the "Fields of Eden", calling in his old boss Bullivant, an expert on old London, to help. They find mention of a pleasure-resort of that name, and Hannay travels to where it stood and finds an antique shop, clearly a front. He arranges a housebreaking expert to let him in by night, and explores, finding the shop backs on to a larger and newer house; he sees a man there he recognises from his time under hypnosis at Medina's, and following the sound of music finds himself in a room overlooking the dance club where he saw Odell, Medina's butler. To his shock, he sees his old friend Turpin there, dancing with his own wife Mary.
He meets Mary later at her aunts' house, and learns she has been working for Sandy, who also knows about the fields of Eden; they have found the missing girl, Adela Victor, disguised as one of dancers at the club, but are no closer to finding the little boy. Hannay is invited to stay at Medina's house, but before he moves there he hears from Mary the Archie Roylance has been to the club, been upset by Odell's treatment of the girl and revealed Turpin's identity. Hannay visits Roylance in hospital, having been beaten by Odell, but Turpin has disappeared - Hannay later learns he was taken, part-hypnotised by Kharama, to a strange house; he shakes off his paralysis, sees his girl in the house, and plays quiet for a time.
Hannay moves in with Medina, whose cool facade has weakened somewhat, and hears that Sandy has been spotted in London. For a week he stays at the house, under close scrutiny, but learns little until he gets a telegram from Gaudian, saying Mercot and his captors have fought and Gaudian has had to lock them all up, bringing Mercot home himself. Tom Greenslade visits, with a message from Mary saying Turpin and Adela Victor are safe. Hannay discovers that Medina's plans to liquidate his gang have been brought forward, and manages to warn Mary.
We learn in an aside of a dowdy but somehow elegant district visitor coming to the house of Madame Breda in Palmyra Square, befriending the maid but being warned off by her mistress. Hannay continues to cling to Medina, until at last Greenslade visits again, and instructs Hannay to come to the dance club the following night, leaving a door to Medina's unlocked, while Mercot travels home by train and Turpin, having awakened Adela from her trance, is held in the mysterious house while she sets off once more for the club, and around Britain, Europe and the world the police close in on a variety of people and places.
Sandy briefly drops in on Hannay and Medina at a dinner of the Thursday Club, from where Hannay goes to the dance club and sees the staff rounded up and Turpin soundly beat Odell; Mary visits Madame Breda once more, leaving with a bundle in her arms and police swarming the house behind her; Hannay returns to Medina's house, and reveals he knows all, pleading with Medina to hand over the boy. Despite the upset to his plans, Medina is cool, but shaken somewhat when Kharama enters and reveals himself as none other than Sandy Arbuthnot, who has looked after Turpin and Adela at Medina's request, and now threatens to expose him if he refuses to heal the child. Lavater, an old friend of Sandy long lost to Medina, is discovered to be Medina's private secretary, and to hold many secrets, but still Medina holds out. Mary enters with the strange little girl from Madame Breda's house; she has become a powerful figure of wrath, and threatens to disfigure vain Medina if he does not cure the child; he gives in, restores the boy's mind, and they return him to his joyous father. Later, Hannay and Mary join Adela, Turpin, Mercot, and Gaudian in celebration.
After a restful spell in the country, Sandy warns Hannay that Medina, never accused of any crimes, may be out for revenge, and advises him to head to Scotland for a spell. Soon he, Mary, their son and Doctor Greenslade are in the Scottish Highlands for the deerstalking season. Soon Archie arrives with news that Medina is at a neighbouring lodge, and Hannay prepares for a showdown. After a long and complex stalk through the crags, Hannay, with no bullets, climbs a dangerous chimney, only to have his hand shot by Medina; he struggles to the top, and Medina follows, but takes a bad turn and gets stuck. He loses his gun, and Hannay lowers a rope to him, but the exhausted Medina is too much for one-handed Hannay to raise up, and lowering him wears through the rope until it breaks, dropping Medina to his death. Hannay falls back exhausted, but is found alive and safe next day.