It’s maddening when even the greatest movies have at least one little plot detail that seems so mind-numbingly stupid that it makes what should have been a perfect 10 into a 9.5.

I’m talking about the one Achilles heel of the original “Alien” — or perhaps I should say Achilles feline. Jones, the cat.

Now, why pick on poor little Jonesy? After all, he can act circles around, for instance, that 9-year-old kid who played Anakin Skywalker in Episode I. He’s also nowhere near as annoying.

Well, it’s not really Jonesy so much as the filmmakers and how they used him to the movie’s detriment.

To its credit, the only gripe I can make about “Alien” is the cat. But I think it’s a legitimate gripe.

First of all, why bring a cat on a deep space mission? Answer: To serve as a plot device in creating situations where certain crew members need to be set up for the monster to get them. Remember Harry Dean Stanton’s exit from the proceedings about an hour in (though, admittedly, that sequence was very well crafted and directed).

But the worst offense comes toward the end. The ship is counting down to self-destruct, the monster’s whereabouts are unknown, and the one surviving crew member — Ripley (Sigourney Weaver) — instead of blasting the hell out of there, runs back into harm’s way in order to find the damn cat.

She does the same thing in the second film, pretty much, but at least in that case it’s a human being she’s going back to find. (Needn’t have bothered, really, considering the opening scenes of “Alien 3.”)

But regardless, the cat thing detracts from the overall logic and intelligence of the story. Because it just defies all common sense. Don’t get me wrong, I love cats, but if I only had five minutes to get off the ship before it went thermonuclear, well … sorry, kitty.

Anyway, it’s a blemish on an otherwise great film. Surely they could have found another way to put Ripley in danger that would have been a little more logical and less insulting to the intelligence.