Family Ties

I think the reason I never saw Family Plot (1976) is that I had it mixed up with The Trouble with Harry. I've figured it out now.

FP starts with two separate threads. In one, we meet Barbara Harris as a medium, doing a reading for a rich old lady. Said lady is tortured by the ghost of her sister, who had a child out of wedlock. The old lady had forced her to give the kid up at birth and knows nothing about him - not even a name. She wants Harris to find the kid and bring him into the family to let her sister find peace. Harris sets her boyfriend, taxi-driver Bruce Dern to find the kid. I was very surprised that they never considered trying to pass him off as the heir.

in parallel with this, a tall mysterious blonde is negotiating with the law for the return of a kidnapped bishop in exchange for a huge diamond. The exchange goes off smoothly, and she is picked up by her boyfriend, William Devane - who looks like Robert Goulet playing Gomez Adamms. She strips off the blonde wig and high shoes, and she is Karen Black.

To cut to the chase (there isn't really a chase scene, by the way), Dern slowly uncovers that Devane is the heir. Devane thinks he is being traced because of his criminal activities, not realizing that he could inherit millions. Instead, he tries to kill Dern and Harris. 

This is more of a comedy than Hitchcock usually goes for, with Dern exasperated by Harris' schemes and super-criminals Devane and Black brought down more or less by mistake. It kind of rambles, with the whole missing heir theme mostly forgotten by the last act. But it keeps moving and is always fun. 

This was Hitchcock's last movie, but far from the last one we haven't seen. Plenty more fun to be had.