As a token of his esteem for Captain George Cooper's prospective father-in-law, Lord Peters, the Rajah of Palakotta presents the captain with a casket of precious jewels. Cooper writes to his fiancée, telling her of this gift he will give her on his return. The notorious cracksman, Clement Rocca, robs the mail-train and reads the letter. He determines to steal the casket. He goes to the English hotel where the colonel and his daughter are staying. He meets them. At the same hotel is detective Harry Derwent. His suspicions are stirred by the action of Rocca. A telegram from Captain Cooper tells the time that his father and himself will arrive. The colonel and his daughter accept Rocca's invitation to join him in a shooting trip on an island off the coast. On arriving at the island, the criminal, on a pretext, leaves his guests. He returns to the mainland, knowing when the tide rises the island will be completely submerged. Not all his own way, however, is the criminal going to conduct his nefarious schemes, for Derwent is on the alert. On the previous night he had seen through a hole bored in the floor separating their rooms that Rocca is supplied with disguises. He had also learned of secret meetings with a man in an out-of-the-way cave, which is accessible only by a dangerous footpath. Capt. Cooper and his father duly arrive and the casket is deposited in the hotel safe. Captain Cooper goes for a walk on the cliffs. Half an hour later he is found insensible. The receipt for the casket has been stolen from the captain's pocketbook. It is Rocca's work. An accomplice, disguised as an orderly, goes to the hotel and is handed the casket on the strength of having the receipt. Cooper and the detective, arriving at the hotel, find that the box is gone. The captain gives chase to an automobile disappearing down the road. The detective hastens to keep watch over the cave. He hides behind a projecting piece of rock. In the interior, Rocca's accomplice hands over the casket. Derwent cautiously proceeds to the mouth of the cave, revolver in hand. He encounters Rocca. Before the latter recovers from the surprise Derwent snatches the precious package from him. There is a struggle. The thief loses his hold on the slippery rocks and staggers back over the edge, into the sea, disappearing forever. The colonel and his daughter, marooned on Bird's Island, have to swim for it when the tide rises. They keep afloat until they are picked up by a fishing smack. They don dry clothes loaned them by sailors and are soon back at the hotel. Captain Cooper returns from the unsuccessful pursuit of the casket. A happy reunion takes place when Detective Derwent enters with the jewels. Written by Moving Picture World synopsis

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