“VENOM” — 2½ stars — Tom Hardy, Michelle Williams, Riz Ahmed, Jenny Slate; PG-13 (intense sequences of sci-fi violence and action, and for language); in general release

Host and parasite aren’t a perfect fit, but “Venom” might have enough of a fun factor to patch most of its cracks.

Ruben Fleischer’s film is an origin story for one of the darker characters from the Spider-Man universe: a kind of black goop parasite that gives its human host superpowers and an impressive set of teeth.

“Venom” opens with a mysterious space shuttle crash in Malaysia that unleashes one of four top-secret alien specimens. Three containers of organic sentient slime are returned to a scientific research foundation in San Francisco, while the fugitive blob starts hopping between human hosts on the other side of the Pacific Ocean.

In San Francisco we meet the leader of the foundation, Carlton Drake (Riz Ahmed) — an ambitious billionaire with some rather obvious Elon Musk parallels — who hopes to use the alien symbiotes to save the human race from overpopulation and climate change. But his unethical methods get him in trouble with “Venom’s” protagonist, Eddie Brock (Tom Hardy).

Brock is an investigative reporter with a strong network backing and a beautiful fiancee lawyer named Anne (Michelle Williams). But he loses it all when he gets too close to Drake’s operation, and is a little gun-shy when a concerned doctor (Jenny Slate) on Drake’s research team lures him back with some hard evidence of her boss’ sins.

Unfortunately, the hard evidence quickly assimilates into Brock. He turns into a kind of black-suited Spider-Man with sharp teeth and unlimited powers of elasticity, and the game is on.

Once Venom shows up — introduced in a thrilling chase through San Francisco — the action level jumps considerably. The film also takes a hard turn into dark humor territory, which makes sense for the subject — the parasite talks to Brock through a kind of telepathy — but feels a bit awkward after the straight-laced first act.

It’s about here audiences would be best advised to focus on “Venom’s” fun factor and dark sense of humor and try to ignore its strange plot twists and clunky expositional tendencies (since the talking parasite gives the script a lot of freedom to tell the audience things it should be showing them).

Venom played a supporting role in Sam Raimi’s third “Spider-Man” film in 2007, but this time around, the black goop alien antihero gets the big stage all to itself. Fans will likely be more satisfied with this outing, though they’ll also likely be disappointed by the lack of onscreen web-slingers.

Hardy does his best to hold the conflicted Brock together, giving him a kind of clumsy goober quality that plays against many of the actor’s more macho characters. The actor has expressed disappointment with the film’s rating, which was originally intended to be R. Even though “Venom” lands at a PG-13, it is quite dark and violent, and the script manages to work in an arbitrary F-word.

Everything builds to a third-act showdown that gives the impression “Venom” will be the first of many adventures that — we would hope — might actually feature “Spider-Man.” Fleisher’s film feels like it’s held together with duct tape, and it’s a step down from the better comic book movie entries this year, but in early October, “Venom” might be enough fun to get a pass.

“Venom” is rated PG-13 for intense sequences of sci-fi violence and action, and for language; running time: 112 minutes.