According to a review in the Times
A good Hitchcock title that. In fact. Fixation, Miles Tripp's story of a young man's fixation for an older woman and the older woman's fixation with the young man and her husband's various fixations might even make a Hitchcock thriller, with a few more corpses added in.
[...]
We were supposed to keep wondering if the youth really did have an unhealthy fixation or if his love for the attractive older woman was only normal. And was the husband himself all that normal? Had not he devoted his youth to caring for his widowed mother and married only later in life? Did he not feel guilty that his father drowned trying to save him? Did he now have a fixation about the permissive society and sex? The husband was a barrel of neuroses. But for some reason, perhaps a very good one, McKenna played him as if he were constantly stifling a yawn.
In the end, which did finally come, although for a while I held little hope for it, the husband threw the kid into the sea (they were on a boat), and the wife then had a fixation that they boy killed himself for love of her while the husband had two drowned men to have flashbacks about.'
Source:
Reynolds, Stanley. 'Fixation', The Times, 18 December 1973, p.9.