Dementia 13
The tagline for this little treasure read "You Must Pass the "D-13" Test To Prepare You for the Horrifying Experience of Dementia 13. If You Fail the Test...You Will Be Asked to Leave the Theater!" and while I question the wisdom of making a film audience sit a written examination pre-showing, we have no such restriction here on the Odeon. (might be worth trying on the next Hollywood Blockbuster though, just for the looks on people's faces)
This 1963 Roger Corman release had a film student brought who had worked as a soundie for him before, on board to tighten up the script, and eventually that student got the directing role too. As a result this is one of Francis Ford Copella's earliest films.
John Haloran dies of a fatal heart attack rowing a boat, but his wife Louise won't get any of the inheritance when Lady Haloran dies if John is dead. Louise dumps the corpse overboard, forges a letter from John to convince the rest of his family he's been called to New York on business, and goes to his Irish ancestral home, Castle Haloran, to meet the family and look for a way to ensure a cut of the loot.