Eddie Brock (played by Tom Hardy) is a slightly sleazy but very good investigative journalist with an eye to helping the homeless. Unable to reign in his own instincts to take down the big guys (in this case Carlton Drake of the Life Foundation, played by Riz Ahmed), he betrays his girlfriend (Michelle Williams) by reading her confidential legal documents and then using them against Drake in an interview. Which terminates both Brock's job and his girlfriend's.
Any fan of Marvel already knows that one of the symbiotes that Life Foundation brought back from space is going to end up attached to Eddie Brock. It's the name - he's Venom, Venom is him. So yeah, that happens. But there's an adjustment period: Venom is intelligent, but violent and nihilistic, which ironically makes Eddie the more responsible of the pair. And since we know that four of the symbiotes came back from space ... well, again, fans of Marvel know there has to be a fight.
Wikipedia says "... received generally negative reviews from critics for its script and inconsistent tone. However, some praised Hardy's performance and the chemistry between Brock and Venom." Hardy is one of the best actors in the world today, and he was really good despite a fairly mediocre script and a weak supporting cast. I was mostly against the movie until the last 15 or 20 minutes, when the back-and-forth dialogue between Hardy and Venom (Hardy also voiced Venom) almost won me over. With the Deadpool-like humour (Venom proposes several times to eat various people and dogs and Eddie has to fight him off) I'm not sure if they can pull off the other movie they're clearly lining up - it's just not a kind of humour you can keep going for an entire movie. But I did kind of enjoy this one in the end.