Dr. Basil Traherne, Major Anthony Crespin and Crespin's wife, Lucilla, struggle out of the wreckage of an airplane which Basil has been attempting to pilot to Crespin's Indian post. They emerge into the strange courtyard of a temple which they discover to be sacred to a huge green idol. They have landed in a remote region of the Himalayas. The Mongolian-featured men who press close about them do nothing to make the English party feel at ease. Basil, a distinguished scientist and excellent linguist, learns from a native who understands Russian that they are in Rukh. The Raja, who has seen the crash, will presently come to deal with the survivors. Lucilla recalls that, while on the plane, she read a newspaper account of three men from Rukh, soon to be executed for murdering the Political Officer at Abdulabad. Supposing that the news has not traveled to this lonely spot as yet, and fearing retribution from the Raja should the story be known, Basil finds the paper, tears out the report, then burns it.
Close inspection discloses the fact that the plane is a total loss. Anthony then berates Basil and Lucilla for having persuaded him to fly from England, but Lucilla reminds him that he was once delighted at the opportunity.
"I thought we'd get to the kiddies a week earlier," Anthony explains. "They'd be glad to see me ... They don't despise their daddy." The Crespins are on strained terms, for the Major has spent most of his leisure either "in his cups" or with other women. "it shan't be my fault, Anthony, if they ever do," Lucilla answers him, "but you don't make it easy to keep up appearances." She is disgusted with Anthony's weakness, though she is also determined to make their marriage satisfy convention and safeguard their children.
The Crespin's marital problems seem unimportant when the Raja, a handsome, polished Oriental, arrives to escort them to his palace. He seems to take perverse pleasure in torturing his unwilling guests. Discovering the mutilated newspaper from which Basil has torn the fatal story, he informs the group that he has already heard that three of his subjects are to be executed--furthermore, the men are his half-brothers. The superstitious natives have decided, he tells them, that the two Englishmen and the lady have been brought to Rukh by the Green Goddess for sacrifice, in payment for the lives of her followers. Obviously, the European-educated Raja does not share this belief, but he is willing to make use of his people's superstition to satisfy his desire for revenge. Basil and Anthony stand for everything he hates in the conquering English race; his absolute power has been diminished through the efforts of such "old Army" men as Anthony and clever scientists like Basil.
The captives are treated with utmost respect and courtesy in the Raja's palace. They know, nevertheless, that their host will kill them in two days, for the Raja has received the news that his brothers are to be executed in that time. Suddenly, the Raja offers Lucilla the choice between becoming his queen, having her children brought to her at Rukh--and death. Lucilla declines his offer, feeling that she can never face her children if she should submit to the man responsible for the murder of their father and her friend.
On the day before the scheduled sacrifice, Basil and Anthony decide that they have but one chance to outwit the Raja. Should any news of their plight come to the outside world, English justice would move swiftly and mercilessly. The news of the execution to take place in Abdulabad has come through a wireless set, concealed somewhere in the palace. If it can be located, Anthony, who is familiar with Morse code, can send a message to the nearest air-drome. Then they can but hope that it will be possible for rescue to reach them before it is too late. Secrecy is important, for, should the Raja discover that he has been duped, he will probably kill the three immediately and destroy all evidence of their presence in Rukh.
The Raja, far from being a fool, realizes that such a thought may occur to the Englishmen. He is not convinced by Anthony's pretence of knowing nothing of wireless transmission. As a test, the Raja takes his prisoners to the wireless room. He has his Prime Minister, Watkins, a renegade Englishman, tap out a message: "The lady has come to terms. She will enter his Highness's household." Anthony, understanding that the Raja is trying to trick him into betraying his knowledge, suddenly gains the necessary poise and courage to deceive their jailer by keeping silent. The Raja, satisfied now that none of the party can send a message, concludes that the only course open to them would be an attempt to bribe Watkins to send a call for help. And the Raja trusts Watkins implicitly, for the renegade is wanted for murder by the English police and therefore cannot betray the ruler of the only place in which he can safely hide.
Basil and Anthony do indeed attempt to bribe Watkins. It amuses the Raja to know that his Prime Minister pretends to accept their money, then takes the captives to the wireless room where he taps out a false message. What the Raja does not know is that Anthony, detecting that Watkins has tricked them, throws him out of the window in the wireless room and works the key himself. But the Raja hears the disturbance in the wireless room and rushes in in time to interrupt Anthony with a revolver shot. "How much ... did you get through?" the ruler asks him. "Curse you--none," Anthony replies, weakly. He dies.
The next day, Basil and Lucilla are led to the temple of the Green Goddess. Anthony's body has been placed at the Goddess's feet. The Raja tells the two that they are to join Anthony--unless, of course, Lucilla wishes to reconsider and to accept his offer of becoming his queen. The victims are left alone, for the Raja hopes Basil will persuade Lucilla to save herself. However, Lucilla refuses to let him die alone. Basil, moved by her decision, tells her that he has loved her for years, but has been silent until now out of respect for her marriage. Lucilla admits that she returns his love. Basil says, regretfully: "We have sacrificed to an idol as senseless as this." He points to the figure of the Green Goddess. Facing imminent death, he and Lucilla suddenly realize that, just as the natives give the idol human lives, as the Raja betrays his knowledge and his people's faith to vengeance, as Anthony gave up decency and happiness to drunkenness and debauchery, so they have sacrificed "all the glory and beauty of life" to convention.
The Raja now returns, to receive Lucilla's second refusal. He reminds her that he has the power to make her a slave if she will not be his queen. Basil leaps at the Raja's throat. The priests, who have followed their leader, seize him, and the Raja tells Basil he has committed an unpardonable offense by laying violent hands upon the man most sacred to the Goddess. Death alone will not atone--the natives will insist that the culprit be tortured. Lucille offers to submit to the Raja if he will save Basil. The Raja agrees to tell the people that the Goddess has spoken and to ask that the Englishman merely be imprisoned--on the condition, of course, that she will be his queen and will not attempt to escape from Rukh.
Suddenly, there is the sound of motors overhead. Looking up, the Raja sees twenty circling airplanes. He realizes that Anthony had lied to save Basil and Lucilla. Anthony's message, then, had been received in time. An English flight lieutenant lands his plane and is escorted to the temple courtyard. He demands the release of the prisoners, but the Raja laughingly reminds the lieutenant that the Rukh soldiers outnumber the pilots of the planes. Every plane overhead is loaded with bombs, the Englishman warns the Raja. Unless a signal is immediately given that the prisoners are safe, Rukh will be blown to bits. The Raja is defeated. He reflects that his position is indeed embarrassing, since he must now account for Anthony's death. He will probably be forced to join other exiled kings.
The Raja looks after Lucilla as she and Basil are escorted to the planes--and safety. Philosophical, unabashed to the end, he murmurs: "Well--she'd probably have been a deuce of a nuisance."