Terry Prescott (Mark Ruffalo: Ride with the Devil, 54) returns home to Scottsville, a small upstate New York town, to sponge some dough off his big sister, Sammy (Laura Linney: The House of Mirth, Running Mates), single mother of 8-year-old Rudy (Rory Culkin). It's been several years since he's been back to Scottsville -- he's been drifting hither and yon, sending Sammy postcards from Florida and Alaska and points in between. She is very excited to see him again -- since they were orphaned as children, each is all the family the other has -- but almost instantly upon their meeting, they fall back into what is obviously an old dynamic: she becomes a bit of an overstuffed prig in response to his rambling irresponsibility and immaturity. Terry decides to hang around for a while anyway.

Written by stage director Ken Lonergan (who also wrote the much more conventional The Adventures of Rocky and Bullwinkle, and Analyze This), You Can Count on Me is his debut as a film director, and what a debut it is. His beautifully perceptive script tells its story in everything that isn't said, from the flashback opener of the eulogy at the funeral of Terry and Sammy's parents, the minister's words unheard by us, to all the unspoken everything that exists between siblings, the unexpressed hopes and dreams we have for each other, the way we rely on each other as a kind of compass to guide us through the mess that the world is. Lonergan's insight into his two very different lost little boys -- Rudy, bereft of his absent father, and Terry, bereft of any direction in his life -- is lovely, as is the clear-eyed way he looks at Sammy. Her life is as screwed up as Terry's, in its own way -- her on-again, off-again relationship with sorta-boyfriend Bob (Jon Tenney) is getting confusing, and her job as a bank loan manager seems under threat from her contentious new boss, Brian (Matthew Broderick: Election, The Lion King Ii: Simba's Pride). But none of that deters her in her quest to set Terry on the straight and narrow.

Lonergan's exquisitely subtle script comes to life, heartbreakingly, through extraordinary performances all around. Young Rory proves himself the least hammy of the Culkin spawn. Linney walks a tightrope of despair, frustration, and exhilaration, Sammy's emotional dilemmas playing eloquently across her face, her delicate features belying her underlying toughness. And Ruffalo is a wonder, by turns petulant and sulking, and explosively passionate -- he uses his expressive, pre-Raphaelite pout to its full measure.

If you have siblings you're close to, you know that not knowing what you want for yourself doesn't preclude knowing what you'd like for your brothers or sisters to achieve for themselves -- it always seems easier to run someone else's life rather than your own, especially when you care deeply for that someone. You Can Count on Me knows this, and explores this particular brand of intimate relationship like no film has before. This is a superbly crafted film, one I won't soon forget.