X,Y&Z stars that romantic trio of Elizabeth Taylor, Michael Caine, and Susannah York. I'm thinking Richard Burton must have been working on another project so Michael Caine was substituted in a part that seemed clearly written for Burton.
Caine and Taylor are a pair of married somethings who are starting to look a lot like George and Martha from Who's Afraid Of Virginia Woolf. That would be another 20 years or so. At the moment they still have a lot of life and lovers to experience.
But for reasons not fully explained Taylor resents York, a widow with two small children far more than any of the others Caine has had. She no doubts sees a slightly younger version of herself. It really is because while the two women are having a most civilized lunch to amicably dispose of the matter of Caine, York says something to Taylor that really reminds her of herself. Something that later Taylor uses to her advantage. At the end we don't really know what's in store for this triangle.
Caine and Taylor have the showier roles, but York gives a nice understated performance. Not one hint of the end I will reveal, but it really does kind of blow Caine's mind.
I'm sure this had Richard Burton in mind originally. Watch and see if you don't agree.