Murder Ahoy (1964)

In my view, this is the weaker of the four "Miss Marple" films made by George Pollock for MGM. Margaret Rutherford is flawless as the trustee of a charity charged with the rehabilitation of young offenders and Lionel Jeffries provides the laughs as the Captain of "HMS Battledore" - the sailing sloop that is used to convert these youngsters from their recalcitrant ways. As one of her fellow trustees suddenly - and literally - snuffs it, "Miss Marple" suspects foul play and with the help of a home chemistry set (and, "Mr. Stringer", of course) deduces that he was ingeniously poisoned! She sets off to the seaside visit the ship and get to the bottom of it all, to the chagrin of virtually everyone concerned. All great japery-by-the-sea and we even get a bit of swordplay.