At the port of Baymouth, American John Nordley, who operates the charter boat the Aloha , takes on a passenger named Mr. Anselm, who wishes to go to Holland. As they approach their destination, John enters Anselm's cabin and finds him dead. He is soon approached by a young woman in a motorboat, who identifies herself as Anselm's daughter and asks to see the body. The woman immediately takes Anselm's money, then flees when John enters the cabin. Suspicious, John, who had earlier noticed an envelope taped to the dead man's chest, removes the document and locks it in his safe. At the port, John is met by a police officer, who takes one look at Anselm and tells John his passenger was actually Inspector Sluiter of The Hague police. John is taken in for questioning by Hoff Commissar Van Der Stoor, who reveals that Sluiter was in London on an official police investigation and should have returned with an important letter. Van Der Stoor then brings in Sluiter's daughter Constanta, who is not the woman who boarded John's boat. Van Der Stoor says that Sluiter, who was diabetic, was murdered when, prior to his boarding, someone replaced his medication with insulin capsules containing ten times the proper dosage. The police hold John as a material witness, but he is released when an anonymous party posts his bail and reserves a hotel room for him. At the hotel, John arranges a meeting with his friend Charlie Ponz, then enters his room and finds a man named Wilhelm Dekker waiting for him. Dekker takes John to see the mysterious Capt. Rohner, who asks him about Sluiter's belongings. John evades Rohner's questions, and as he is walking out, Elsa, the woman who boarded his ship earlier, comes into the room. John goes to a waterfront bar and meets the disreputable but charming Charlie, and asks for his help getting past the police guarding his boat. Back at his hotel, John gets a call from Elsa and goes to her apartment. Elsa tells him Rohner ordered her to go aboard his ship and look for an envelope, adding fearfully that Rohner believes she is withholding the envelope from him. John notices Dekker outside and accompanies him to see Rohner, who says Elsa was the one who switched Sluiter's insulin capsules. Rohner then explains that he was in charge of the garrison during the Nazi occupation, and made a pact with a banker named Maserling to smuggle funds into Berlin. The launch carrying the funds was sunk by Dutch partisans and Maserling, who was disguised as a German officer, was taken into custody by the English. Rohner says that before his capture, Maserling had prepared an overlay showing the location of the sunken treasure. He adds that Maserling died the previous week and Sluiter, who had been working on the case for years, made a deal with Rohner to deliver the overlay to him. Rohner offers John a large sum of money for the overlay, but John demands Sluiter's cut. John then returns to Elsa's apartment and finds her murdered. He immediately has Charlie accompany him to the Aloha , and while Charlie distracts the police, John sneaks aboard and opens his safe, only to find the envelope containing the overlay missing. John calls on Constanta and discovers that she has the envelope in her purse. Constanta explains the information on the overlay for him, identifying the chief landmark as an island known as the House of the Seven Hawks. John and Constanta arrange with Charlie to sail at dawn in search of the sunken ship, but after Charlie watches the couple kiss outside the bar, he calls Van Der Stoor and informs him that John plans to leave Holland. Charlie than places a call to Rohner. The following morning, John, Charlie and Constanta take a boat and locate the launch, but before John can remove the strongbox from the cabin, Rohner arrives with Dekker and his henchman Pieter. Rohner decides to wait for low tide to have Pieter dive, so Charlie takes John and Constanta to the island and holds them hostage in a farmhouse inhabited by a kindly elderly couple. John notices a clock in the house bearing the name of the launch and asks the farmer, Beukleman, where he got it. The old man replies that near the end of the war he took in a wounded German captain, who told him about the launch. Beukleman admits he removed valuable objects from the launch, feeling entitled to reparations for the damage the Germans inflicted on his farm. John relates Beukleman's story to Charlie, pointing out that the "German captain" was Maserling, and offers to split the fortune with Charlie if he will shift allegiances again. The pragmatic Charlie agrees and knocks Dekker out. Buckerman shows John and Charlie where he has buried the treasure, and they are surprised to find bags of diamonds. Rohner and Pieter return empty-handed from the launch and engage in a gunfight with John until the police arrive. John, Charlie and Constanta escape the island and go to Van Der Stoor. John hands over the diamonds, admitting to the astonished Charlie that he had been cooperating with the police all along. Van Der Stoor releases John's ship and offers him a large reward. When John declines the reward, Van Der Stoor says the money will go to Constanta. At the port, John asks Constanta to return with him to Baymouth, and she happily agrees.
