Opening title card: "The slow shadow of death is falling on the outer isles of Scotland. This is the story of one of them - and of all of them. When the Roman fleet first sailed Britain they saw from the Orkneys a distant island. Like a blue haze across a hundred miles of sea. They called it - 'Ultima Thule'".
Two tourists yachting in the Hebrides decide to stop on the now deserted island of Hirta, against the advice of Andrew Gray, their guide. While exploring the island, the yachtsman and his girlfriend find a gravestone bearing the inscription 'Peter Mason - gone over'. They prevail upon Andrew to tell them the story behind it. Andrew explains that ten years earlier the island had still been inhabited and that he was engaged to Ruth Manson, the sister of his best friend Robbie.
One Sunday, after mass, Andrew, Robbie and Ruth meet on the cliffs. Robbie, who has just returned from a trip to the mainland, shocks them by announcing that he plans to return there to marry, and to remain there, despite the fact that they're already short-staffed on the island.
Robbie is convinced that living on the island is no longer economically viable, but Andrew sees this as a betrayal and suggests that they settle the matter following an ancient ritual: a race to the top of a sheer cliff face. Whoever wins will decide the fate of the islanders.
The following day, at a meeting of the parliament, Robbie explains his objections, and is backed by Andrew's father James, but Robbie's own father, Peter, angrily rejects his son's point of view and agrees that the challenge go ahead. The men head for the bottom of the cliff, where Robbie and Andrew set off, taking different routes. Andrew warns his friend not to take a short cut that is too dangerous to attempt as they are without ropes. Andrew soon takes the lead and Robbie attempts to take the short cut, but soon becomes stuck at a small waterfall. Having won the race, Andrew runs to Robbie's aid when he realises what has happened, as do the rest of the men. Andrew almost reaches him in time, but Robbie falls to his death.
At the funeral, Peter accepts James' sympathy, but shuns Andrew. In the following three months, Ruth and Andrew find it hard to meet as Peter won't have anything to do with him. On what would have been the day of their wedding, Ruth and Andrew find a secluded place and spend an afternoon together. Ruth, however, feels that she must do as her father says, so Andrew reluctantly asks for the help of the Laird when he comes to collect the harvest, but to no avail. Andrew is forced to leave the island.
Some time later, Ruth, lacking any other means of communication on the island, sends a message attached to a float out to sea in the hope of it reaching Andrew, but Peter intercepts it. In it, she says that she is pregnant with Andrew's child - Peter confronts her, and faced with the truth, embraces her and forgives her.
For months Andrew, unwilling to work on the steam trawlers that are taking away part of Hirta's livelihood, finds work hard to come by. After the birth of the baby, Peter and James send out a number of mail boats attached to floats in the general direction of the trawlers in the hope of getting a message to Andrew, one of which he eventually receives.
As Andrew heads out to Hirta in a trawler in the middle of a huge gale, Ruth is told that the baby has diphtheria, but that it can only be saved if taken to the mainland. Andrew arrives just in time and is able to take Ruth, James and the baby back to the Scottish mainland, so saving the baby. He vows that he will not take them back to Hirta.
Peter writes to the Laird on behalf of the crofters of Hirta asking that they be evacuated. Soon the day comes, and all the inhabitants load on to boats to leave. Shortly before he is due to go, Peter heads off to a cliff to get an egg that he can sell for a good price to a collector on the mainland. Having found it, he tries to make his way back up the cliff, but Peter falls to his death when his climbing rope becomes frayed and snaps.
