This trifle has hints of The African Queen, with Peter O'Toole as a manic Irish seaman who teams up with a Quaker missionary (O'Toole's then wife, Sian Phillips) and a French oil engineer (convincingly played by Philippe Noiret) to sink a German U-boat on the Orinoco delta. Shot in the jungles of Venezuela, the film's blend of whimsy and battle is skilfully handled by director Peter Yates, who was riding high at the time on the success of Bullitt.

Murphy is the sole survivor of his crew, that has been massacred by a German U-Boat in the closing days of World War II. He is rescued, and ends up at a forgotten mission station near the mouth of the Orinoco, and begins to plot his vengeance. He wishes to sink the U-Boat by means of any method imaginable to him, and sets about to make the courageous attempt, assisted by Louis, the administrator of the local oil company.