The contents of a shark's stomach contains a bottle that holds notes written in three different languages. Much of the notes are indecipherable; however, together they may reveal the location of the whereabouts of Captain Harry Grant, whose ship the Britannia was lost over two years ago. While the latitude of Grant is known from the note, the longitude is a mystery. Clues from the notes point to the South American coast as the probable location of the shipwreck. Lord Glenarvan makes it his quest to find Grant; together with his wife, Harry Grant's children (Mary and Robert) and the crew of his yacht the Duncan they set off on the chase. An unexpected passenger in the form of French geographer Jacques Paganel joins the search.

The search takes them where the 37th degree of south latitude crosses the South American continent. Crossing the continent from west to east, they travel through Chile and Argentina (Patagonia), with no luck, when Paganel realizes that part of the clue must actually mean Australia. So they go to where the 37th degree begins crossing Australia, there by chance they meet a man who claims his name is Thomas Ayrton, former quartermaster of the Britannia. Ayrton takes them across the Australian continent, with plans of his own to seize control of the Duncan. Ayrton's treachery makes the party believe that he has gained control of the Duncan. Dejected that their search of Australia did not turn up Captain Grant, the adventurers travel to New Zealand via a ship that barely seems sea-worthy. The vessel wrecks along the New Zealand coast. The party's attempt to get to Auckland gets them captured by the Maoris, a tribe of cannibals at war with the English and disinterested in keeping the prisoners alive.