Every year the great, swelling sea of summer movies is supposed to part to reveal a small film with a strong independent character, a movie that will seem, amid all the hype and the spectacular dross, to lead the way to the promised land. The authentic “sleeper” is hard to come by, but when it’s there you will know it by its quirky characters, its nice writing, its downbeat feelgood factor, its success at the Sundance Film Festival, and the way it uncovers fresh potential in a group of interesting actors. Even anti-formula movies have a formula.

It seems to me that Sunshine Cleaning will be the sleeper of this year, a movie soon to be talked about in the same breath as Sideways, Juno and Little Miss Sunshine. The director is Christine Jeffs, who drew poetry out of childhood once before, in Rain, and poetry out of poetry in Sylvia, her biopic of Sylvia Plath.

This time around, at the instigation of brilliant first-time screenwriter Megan Holley, she tells the story of the Lorkowski sisters, who are trying to make their lives work in a smallish American town. The elder, Rose (Amy Adams) is having an affair with her married high school sweetheart, a local police officer, while trying to get her odd son Oscar (Jason Spevack) — he licks furniture — into a private school. Rose’s younger sister Norah (Emily Blunt) is your classic, heavily eye-linered, middle-finger-wielding hardnut who is actually quite soft inside.

They all have issues, needless to say, but the issues are subsumed very naturally into the other stuff that happens in life when you’re not going on about your issues. That means fun and romance, enterprise and absurdity.

What you get with Sunshine Cleaning is all of the above and a little bit more. The film is a small triumph. Rose was once her school’s head cheerleader, but now the years are running away from her: she needs money to help her son and she and Norah find it in an unexpected place, working in crime-scene clean-up. After the police have done their work at the site of suicides, murders, and lonely deaths, the girls go in with their mops and their disinfectant. It probably sounds like an unlikely scenario from which to expect warm humour and terrific insights into the nature of human empathy, but that’s exactly what you get — an object lesson in how to conjure laughter out of the dark.

Adams and Blunt are brilliant together. The older one brings the prettiness and the sparky hunger for life’s big dreams that made her so good in Enchantment. She also brings the sharp instinct for uncertainty that made her the best thing in last year’s Oscar-nominated nun-fest Doubt. Emily Blunt knows how to reveal the shaky foundations of her character’s rebellion: Norah is a bit hopeless, but she cares about her sister and their shared secrets of childhood.

As the film goes on and the laughs build, you feel the closeness of the sisters emerging out of their trials. Alan Arkin plays their dad with a beautiful combination of pride and delusion: he too has his schemes for improvement, and they often fail, but the atmosphere of the movie is so lovely and humane