Act One is set in Loam Hall, the household of Lord Loam, a British peer, Crichton being his butler. Loam considers the class divisions in British society to be artificial. He promotes his views during tea-parties where servants mingle with his aristocratic guests, to the embarrassment of all. Crichton particularly disapproves, considering the class system to be "the natural outcome of a civilised society".
At the beginning of Act Two, Loam, his family and friends, and Crichton are shipwrecked on a deserted tropical island. The resourceful Crichton is the only one of the party with any practical knowledge, and he assumes, initially with reluctance, the position of leader. This role begins to take on sinister tones when he starts training Ernest, one of the young aristocrats with them, to break a liking for laboured epigrams by putting his head in a bucket of water whenever he makes one. Crichton's social betters at first resist his growing influence and go their separate ways, but in a pivotal scene they return, showing their acquiescence by accepting the food Crichton alone has been able to find and cook.
Act Three reveals the island two years later. Crichton has civilised the island with farming and house building and now, called "the Guv.", is waited on with the trappings and privileges of power, just as his master had been in Britain. Lady Mary, Loam's daughter, falls in love with him, forgetting her engagement to Lord Brocklehurst at home. Just as she and Crichton are about to be married by a clergyman who was shipwrecked with them, the sound of a ship's gun is heard. After a moment's temptation not to reveal their whereabouts, Crichton makes the conventionally decent choice and launches a signal. As the rescuers greet the castaways, he resumes his status as butler.
Act Four (subtitled "The Other Island") is set back at Loam Hall, where the status quo ante has returned uneasily. The Loams and their friends are embarrassed by Crichton's presence, since Ernest has published a false account of events on the island, presenting himself and Lord Loam in key roles. Lady Brocklehurst, Lord Brocklehurst's mother, quizzes the family and servants about events on the island, suspecting that Lady Mary might have been unfaithful to Lord Brocklehurst. The household evades these questions, except for a final one when Lady Mary reacts with shock â "Oh no, impossible..." â to the suggestion that Crichton might become butler at her married household. To protect her, Crichton explains the impossibility is due to his leaving service, and the play ends with his and Lady Mary's regretful final parting.

Question: What stops the wedding of Crichton and Lady Mary?
Answer: The sound of a rescue ship's gun.
Alternative answer: The sound of a ship's gun.