Some movies are all about chemistry, and “The Guilt Trip” has enough to fill a lab. Seth Rogen plays Andy Brewster, a would-be inventor desperate to peddle his new cleaning product; Barbra Streisand is his mother Joyce, who accepts his impromptu invitation to join him on a cross-country road trip. They are — who knew? — a match made in movie heaven. He’s sweetly abashed; she’s lovingly bossy and likes to smooth down his hair with a just-licked hand; and they rat-a-tat at each other like, well, mother and son. A scripted Hollywood version of mother and son, to be sure (written by Dan Fogelman, screenwriter of “Crazy Stupid Love”); but still, they’re funny, and you miss them when the movie’s over.

Though Joyce thinks she’s coming along to keep Andy company as he pitches his product to corporations in various cities, the trip has a secret purpose: Andy’s hoping to reunite his long-widowed mother with an old flame, who lives on the opposite coast. But though this plotline gets a resolution that had a preview audience audibly cooing, it doesn’t really matter: “The Guilt Trip” exists for Streisand to bark momly wisdom at Rogen, which she always manages to make funny. Asked why she won’t put her purse on the floor for fear of germs, but will use the same tissue for three days, Joyce retorts, “There’s always a clean side,” and Streisand manages to make it sound at once like a comic gem, a heartwarming philosophy of optimism and a serene acceptance of contradiction.

On “The Guilt Trip” goes, ending before you get tired of it (it’s a tidy 93 minutes) and before Rogen runs out of variants on his tolerant/exasperated straight-man expression. Though slight, it’s a rare contemporary comedy with almost no off-color humor (yes, you can take your mom) and a genuine warmth toward its characters. “Where’s your coat?” Joyce demands, when Andy tries to leave the car in a snowstorm. “I don’t need a coat,” says Andy, probably for the 500th time in their relationship. “Wear your galoshes!” she says, not listening. A familiar song, but it’s a pleasure to hear them sing it.

Moira Macdonald: or 206-464-2725.

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