What might be evidence of a long-buried crime is the mystery at the center of Joe Swanberg’s newest, but it eventually proves a red herring. Ultimately “Digging for Fire” isn’t about uncovering a body as much as it’s about the compromises needed to make a marriage work. It’s an interesting sleight of hand, but unfortunately in this case the execution is so uncertain that it doesn’t come off.

Jake Johnson and Rosemarie DeWitt play Tim, a gym teacher, and Lee, a yoga instructor, who, along with their toddler Jude (Jude Swanberg), arrive at a plush rural estate in Malibu that a client of Lee’s has invited them to stay in while he’s away. The seclusion is supposed to give Tim the opportunity to do their taxes, but he ambles about the place instead, and in the process finds a rusty old pistol and what might be a human bone. He contacts the police, but they’re not impressed: if he locates a body, fine, but otherwise he’s on his own. Though he wants to do some digging to see if he can find more evidence of foul play, Lee protests and makes him promise to lay off the shovel while she goes off with Jude to visit her mother for the weekend.

In his wife’s absence Tim decides to invite over some buddies for a barbecue—uptight Phil (Mike Birbiglia) and rowdy Ray (Sam Rockwell) and even rowdier Tango (Chris Messina)—along with a couple of girls, Max (Brie Larson) and Alicia (Anna Kendrick). He makes the mistake of mentioning his find to them, which leads to their undertaking a major excavation, though Phil counsels against it. And while most leave when the dig proves fruitless, Max returns the next day to continue the effort; eventually they go out to dinner, too. Tim doesn’t even cease when a neighbor (Tom Bower) happens by and blandly suggests, over a cup of coffee, that sometimes it’s better to leave sleeping bodies lie.

Meanwhile Lee is having an adventure of her own. After talking with her mother (Judith Light) and stepfather (Sam Elliott) about the sacrifices one has to make in a marriage, she goes off to spend some time with an old friend, Squiggy (Melanie Lynskey), and her husband Bob (Ron Livingston), but later wanders into town, where she meets a handsome stranger named Ben (Orlando Bloom) in a bar and, after he steps in to protect her from a drunk, tends to his wounds and joins him for a spin on his motorcycle. By the time she returns to the house, Tim has uncovered more evidence of nefarious goings-on in the distant past, but whether he’ll choose to do anything with the material he’s so laboriously unearthed is another matter. (No word about how the place’s owner reacts to the piles of dirt they leave behind.)

Despite the putative corpse in the backyard that Tim is obsessed with, the real skeleton in the movie’s closet is the fact that his marriage, while hardly on the rocks, is nonetheless showing signs of strain, with both husband and wife disagreeing on many things—not just Tim’s investigation but the suggestion that they should accept money from Lee’s mother so that they could send Jude to an expensive preschool—while both come perilously close to infidelity. The movie concludes with the suggestion that sometimes it benefits people not to know the whole truth about one another and that a bit of discretion might not be the worst thing in a marriage.

Swanberg brings a genial looseness to “Digging for Fire” that can be engaging, and certainly both Johnson and DeWitt are agreeable performers who give their characters a good deal of likableness. Some of the “guest stars” strike sparks too, with Bloom coming off especially well (though Elliott and Rockwell have little chance to shine, and Kendrick plays a blink-and-you’ll-miss-her part). And the cinematography by Ben Richardson adds considerable atmosphere, especially in the nighttime scenes.

Overall, though, the picture represents a decline from the prolific Swanberg’s previous effort, “Drinking Buddies,” which was more crisply plotted and played. In the end the looseness turns into haphazardness, and while the picture digs around a lot, it never really catches fire. By the close you might feel that the real mystery is why you should care.