Lieutenant Sylvester Green (Donald Sinden) is making his crooked way shipwards with a couple of Naval colleagues after a night's drinking, and along the way, they collect an old pram and three brass balls from a pawnbroker's shop. Passing the foreign vessel Agraria on the dock, they notice some welding equipment and decide to have some fun. They attach the junk to the Agraria and paint it Naval grey. The next morning, when Green unsteadily arises, his commanding officer has noticed the mysterious new equipment, and trying to avoid exposure, Green identifies it confidently as a "998". From then on, things only get more confused, and soon everyone in the Navy wants to know why this foreign ship has more up-to-date radar than they do. Green is coincidentally seconded to the Agraria as a radar instructor and soon (through the magical power of coincidence) the Agrarians (and everyone else) believe it is a powerful secret weapon. Green is held prisoner in the rather pleasant environment of the Agrarian palace where the President's (Akim Tamiroff's) many beautiful daughters have been instructed to fulfill his every desire. Eventually his girlfriend Betty (Sarah Lawson) (also in the Navy) and Lieutenant Smart (Bill Kerr) come to Agraria to rescue him, but all is settled amicably in the end, and the Agrarians get to manufacture their own 998's out of junk as a deterrent to their neighbors.