A megazillionaire is sponsoring space exploration for his own nefarious purposes. Mankind is destroying the planet, so he plans to set up shop Out There somewhere so he can live like a space emperor. Or something; I may not have been paying attention when the evil plot was explained.
But one of his probes returns with deadly aggressive metallic snot creatures, some of which immediately get out of control. They are "symbiotes", who like to glom onto humans, infiltrate their bodies, and take over, not always successfully. The bad guy sees them as someone he can do business with, and sets about experimenting with human subjects. Who are not providing informed consent. Results are discouraging and disgusting.
But the overall goal is for the bad guy to bring back lots of these creatures to Earth and take over.
Enter our hero, Eddie Brock (Tom Hardy). Fired from his investigative journalist gig because he tried to investigate the wrong guy (our villain), he blunders into the lab/lair where the aliens are being held and… oops, he's taken over. Fortunately, he survives, and develops a complex relationship with his symbiote, aka Venom. And they mutually decide to do that thing heroes do: defeat the villain's plot.
It's even more ludicrous than the usual comic book movie, but maintains a surprising amount of humor. Venom and Eddie have a contentious relationship and bicker a lot. Remember The Odd Couple? Yeah, like that.