The film was released following the end of the second season of the television series.
Plot
Set "Somewhere in the South Pacific" in 1943, Gruber's gambling scheme backfires when he tries to raise money for St Theresa's Orphanage through off-track horserace betting. Heavy bets on the horse "Silver Spot" leaves the crew owing a large sum to sailors and marines. Captain Binghamton is attempting to spy on the illegal operation from the air, but Ensign Parker mistakes his plane for the enemy and shoots it down. The Captain is rescued from a wet landing in the bay, but can't restrict McHale and his crew to base because they have been ordered to report to New Caledonia for a special briefing.
Some time later, when the crew is in New Caledonia, Ensign Parker runs PT-73 into the dock and destroys the dock and cargo of businessman Henri Le Clerc, leaving the crew even more in debt. However, while on a reconnaissance mission to an island the crew comes across Silver Spot who was lost on the island after the horse was being moved from Australia. They decide to enter Silver Spot in a race in New Caledonia to win enough money to pay all their debts. Otherwise, the only way to pay all their debts is for McHale to marry his old flame Margot Monet, also known as Maggie Monahan, who owns a gambling parlor in New Caledonia (but who McHale would rather not marry). They try to disguise the horse with much heavy hair, but when the hair starts to come off during the race they decide to use a smoke screen from the 73 to keep anyone from seeing too much. However, this made it impossible for the racetrack authorities to know who won. But the smokescreen also caused a Japanese submarine to be captured by the 73.
In the end, Le Clerc is so grateful for saving his town he forgives the debt. The crew also receive a reward for rescuing Silver Spot which just happens to be enough to pay the sailors and marines. While all this was going on the PT-73 crew was dogging Binghamton and his aide Carpenter. And just as they were leaving New Caledonia the bashful Parker is kissed by Bouchard and the lightheaded Parker accidentally sets off a depth charge which destroys Le Clerc's dock and cargo again. But rather than marry Monet to pay for the new damage, McHale and crew immediately scramble to get out of New Caledonia.