If you’re a guy like John, John C. Reilly’s character in “Cyrus,” discovering Red Bull and vodkas after seven years divorced, and a sparkling, beautiful woman like Molly (Marisa Tomei) comes along and is fascinated by you, sausage curls bouncing with every twirl, yes, it’s apt to think that there’s going to be some kind of crazy waiting for you.

John’s the kind of guy whose ex-wife, played by Catherine Keener, still has a key to his apartment and a position on his speed dial. She walks in on him wanking, with loud music shaking the walls of his apartment, while he wakes her and her long-suffering fiance in the middle of the night to gossip. Invited to a party — the dysfunctions of their dead relationship are such that she still needs to shove him out of the nest — he gets drunk and is shot down in devastating, demure fashion by a woman who looks like she’s spent the 10 years since she was a cheerleader eating her feelings in Double Downs.

So yes, it would be too prime-time TV for John to meet a great, beautiful, normal girl. Molly’s flaw, as we soon find out, is her son, the titular Cyrus, played by Jonah Hill (“Superbad,” “Knocked Up,” “The 40-Year-Old Virgin”). He’s obese and 21 (nearly 22), so when he first shows up, he comes off a bit like Molly’s husband or lover. He refers to her as “Molly,” after all, and swans about the house with the confidence of an Orson Welles, even dressed as he is in the pressed khakis and striped polo shirts of a student council member.

“It’s nice to have a new dad,” he deadpans. But he’s kidding! Or is he? (And then there’s the scene where he shows up naked, wearing a large, soiled shirt, no pants, waving a knife.) Molly, after all, doesn’t keep the door closed. They’re just that kind of house. And Cyrus will stick his head in the shower when she’s bathing. What?

Cyrus is the elephant in the room: it’s a classic, midcentury California greatroom and he’s playing serious, synthesized, Hearts of Space soundscapes.

Something happens around the middle of the movie. The three main characters — John, Molly, Cyrus — experience events that seem to lead to a transformation. They have to behave themselves around Catherine Keener’s character’s new wedding. Cyrus is making overtures about moving out, getting on with his music style. In gorilla style, John and Cyrus basically throw down the gauntlet for the beautiful Molly. But that’s all she remains. Molly is a stupid pawn. It’s funny because she’s beautiful, right? And a beautiful woman wouldn’t normally have her fate batted around for play by an ugly man and a fat kid, but it is now, ha ha. And it’s okay to laugh, because they want the best for her. And it’s okay, because they don’t mean any harm. And it’s okay because they threaten each other with violence as she sleeps.

As California movies are, it’s a bit talky and sensitive and uncomfortable, like a “Curb Your Enthusiasm” episode with no crotchety New York guy to lighten the tension. Everything’s very serious, because these are people who spend weeks irritated over a moody transaction buying sushi at Whole Foods, and they have no real problems. The way John and Molly try to dig out a place for their relationship is rather beautiful and different, as he describes himself as “borderline desperate,” sings that bad Human League song at a party and she joins in, not wanting him to look like an idiot, and he follows up with, “am I supposed to be cooler than that?” after getting her to his place twice in a row.

It’s funny. It’s different. It’s dark. There’s still love in it. That’s why it’s disappointing, in a movie that bucks so many traditions in a rut of a genre, that the female lead ends up stupid, unchanging, ditzy, stupid, getting along on her looks and nothing else, a fool. She’s not a character, to interact with these two doofuses. She’s a prop. With so few changes, this could have been so different.

Cyrus says this, handing a pitiful copy of his CD-burned album to John near the end: “It’s mixed. But it’s not mastered.”

Ashley O’Dell writes about movies that aren’t in theaters anymore.