Extremely hard to review: the middle is magnificent, but the ending is terrible

I've encountered many films that started strong and then failed to stick the landing. Less often, I've seen movies that started poorly (or at least "mediocrely", if that's a real adverb), and course-corrected to finish triumphally.

But this movie starts out for roughly the first 30-40 minutes as a solid B- '70s noir, an update of the private eye genre in the Rockford Files vein, that has now itself become transformed into an artifact of a bygone era by the passage of time.

Then there is a middle section, set in the Florida Keys, that is absolutely magical. Not a lot happens (although it turns out there are some silly crime shenanigans lurking in the background), but the character work, the dialogue, the acting, all perfectly tuned into the setting, cast a hypnotic spell. People say things you never hear in movies, but there's the instant glint of recognition, that it's true to life. They even stammer their words occasionally, without being "stammering" characters or in a particularly stressful situation. They just stumble over what they are trying to say, as we all do sometimes, and it was either an accident by the actors the filmmaker decided to leave in, or a masterful exercise in purposeful cinema verite. Either way, brave and bold by the director and/or editors. This section, on its own as a short film, would get an A from me--maybe even an A+.

Then in the final act, part of which again takes place in Florida, there is a quick succession of murders, some melodramatic speechifying, a series of increasingly absurd action set pieces, and a number of hairpin "twists" that land with a thud. D-.

So what does it all add up to? Hell if I know. C, I guess? But it is not like any other grade C movie I can think of; and it was extremely difficult to rank for the same reasons.

If you ever watch this, I can see now in retrospect the last moment when the film held some hope of being something of real quality as a whole. Get out after this scene between Harry and his estranged wife Ellen at the airport, and it won't be as badly tainted by the pile of nonsense at the end. It also provides an taste of some of the clever dialogue in the script, before it goes brain-dead:

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HARRY: Now, look, l know you've been alone a lot, even when l was around.
And l know when you get....
When we get like that, we reach out for other people.
ELLEN: Marty was a distress signal. You were passing by at the time.
HARRY: l didn't mean just you.
ELLEN: l know what you didn't mean. [Gives Harry a meaningful look]
You're gonna miss your plane.
lf you don't go now, you can't come back.
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"I know what you didn't mean". That is so great, so much better than simply having her say "I know what you meant."

And then it turns to shite.