That little square box

Mr. Hammond is on his way to America when he hears two passengers discussing a suspicious plan that was going to be executed at 10 o'clock in the evening. The two unknown passengers got aboard in the very last minute. Their baggage was not searched. And their plan consisted of some mysterious box. What do you think happened in the evening? Did Mr. Hammond try to warn the rest of the crew or he escaped with a life boat? Can all be a big misunderstanding or the American steamer will be blown up?