When Lord and Lady Glenarvan catch a shark during a sailing trip, they are surprised to find a message in a bottle within its belly, sent by Captain Grant of the shipwrecked Britannia. Unfortunately, the water-damaged note is mostly unreadable, giving any would-be rescuers only a latitude and no longitude to work with.

Eager to help the castaways, the couple urges the British government to launch a rescue expedition but are refused. So they take it upon themselves to search for Grant with the help of Mary and Robert--Captain Grant's children--and the crew of the Duncan. Thus, the group sets off, determined to find the shipwrecked crew, even if it means sailing across the entire thirty-seventh latitude--and facing whatever dangers lie in their path.