The book tells the story of the quest for Captain Grant of the Britannia. After finding a bottle the captain had cast into the ocean after the Britannia is shipwrecked, Lord and Lady Glenarvan of Scotland contact Mary and Robert, the young daughter and son of Captain Grant, through an announcement in a newspaper. The government refuses to launch a rescue expedition, but Lord and Lady Glenarvan, moved by the children's condition, decide to do it by themselves. The main difficulty is that the coordinates of the wreckage are mostly erased, and only the latitude (37 degrees) is known; thus, the expedition would have to circumnavigate the 37th parallel south. The bottle was retrieved from a shark's stomach, so it is impossible to trace its origin by the currents. Remaining clues consist of a few words in three languages. They are re-interpreted several times throughout the novel to make various destinations seem likely.
Lord Glenarvan makes it his quest to find Grant; together with his wife, Grant's children and the crew of his yacht, the Duncan, they set off for South America. An unexpected passenger ...