Detective Lambert Chase is on his way to Europe: he is surprised to see a box lowered from a ventilator, leading out to the upper deck. He takes the box, and discovers it contains a set of jewels. He puts them in his pocket, says nothing about them. Later, reports are circulated about the steamer that many articles of value are missing from the staterooms of the passengers. Chase goes to the Captain, confides his suspicions and tells him the incident of the box. They investigate, and locate the ventilator. They set a guard there, who watches for the reappearance of the thief. Among the passengers are Mr. and Mrs. Carrell, wealthy people, who have with them their maid Jane. Chase suspects her of being an accomplice of the thief, whom circumstances point out as Linten, a suave, refined, cultured rascal who makes himself very popular among his fellow-travelers. The maid is quietly arrested and locked in one of the cabins. When the rumor about the thefts being committed, is circulated, he is sympathetic with those who have been robbed, and shows great interest. In his arrangements for capturing the thief, Lambert Chase ties one end of the string, which was lowered from the ventilator, to a small bell, safely hidden on the lower deck, where he and the Captain await developments. This is done on the last day out, when Chase is certain the thief will get in his best work. His plans are rewarded by hearing the bell ring, then, after a pause, ring again. This convinces them that the thief thinks the string has caught. The Captain and the detective hasten to the upper deck, and there they find Linton striving to pull up the cord. They grab him, and after a search, find his pockets filled with many stolen articles. He is handcuffed.