During a drought, Margaret's father must reduce the family's expenses. Margaret dreads leaving Wirra Wirra, the private school that she loves, to attend the local high school. But her generous eighteen-year-old brother, Bob, gives up his University studies in medicine to join the Air Force.
After Bob's death at the front, Margaret meets a naval friend of her brother who invites her to tour his ship. Margaret suffers a fall and lies unconscious in the hold for two days until the ship is torpedoed and she wakes to find herself in a life boat in the Pacific Ocean. Shipwrecked with five naval castaways, Margaret applies all of the courage and faith she learned at 'Wirra Wirra'. When they reach an island, she is surprised to find that Bob has not been killed in battle, but made his way to an island after his plane went down. Bob uses his medical knowledge to save a colleague and they are finally rescued by an American battleship.
Bob brings back with him a sample of berries that arrest haemorrhage and receives a hefty monetary reward that enables him to return to University to become a doctor. Margaret is named Head-Prefect at Wirra Wirra.