Ten Cents delivers his last cargo barge to the Old Quay after a very busy day. As night falls, it is towed away by a mysterious boat. Captain Star is cross to hear that one of his customer's deliveries is missing and puts Ten Cents on dredger detail - not a pleasant task. But Grampus, a small submarine, sees another barge being stolen at night, so Captain Star and Captain Zero join forces to find the thief....

It has been one of the hottest weeks of the summer. In such hot weather, there was a greater danger that ships might be carrying diseases. If this was the case, the ships would have to go into quarantine for several days and wait out in the estuary, away from the harbour. Under no circumstances should any of the tugs go near the ships in quarantine. Ships in quarantine fly the yellow flag. When the old tramper, Nantucket, lowers its flag, a great deal of confusion is caused and only wise old O.J. understands why.