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...