For 30-year-old Jeff (Jason Segel), the universe is sending him subliminal messages, and once he's deciphered the meaning of them he'll be ready to move out of his mother's basement.
For his older brother Pat (Ed Helms), the universe (with help from his wife) is conspiring to deny him happiness and fulfillment, a state of affairs that can only be resolved by buying a Porsche he cannot afford.
For Jeff and Pat's widowed mother Sharon (Susan Sarandon), the universe has condemned her to a life without love and the growing suspicion that her two sons are idiots.
Like planets in opposition, the three are locked in their orbits, and only their mutual gravitational pull prevents them from spinning into the void.
In Jeff, Who Lives at Home, we are invited to spend a day with these three — the very day they all learn hard lessons about their real places in the cosmos. As the hours tick by, we progress from being utterly unimpressed with them to laughing at their foibles, then sympathizing with their anxieties, and finally rooting for them to recognize the absolute rightness of where they are, right now.
It's Sharon's birthday, and when she calls her slacker son from the office, she has just one request: that he buy some wood glue and fix a broken slat on a door at home. Jeff promises to do so, but there's a problem: His entire philosophy of life is modeled on M. Knight Shyamalan's sci-fi movie Signs — which, you may remember, involved a long series of far-flung coincidences that enabled Mel Gibson to defeat alien invaders. In other words, the slightest thing — a half-filled glass of water, a tossed-off phrase, a souvenir baseball bat — can in the end have the most profound implications in the course of your life.
It's a philosophy that has, in fact, left Jeff utterly paralyzed, looking for meaning in just about every little thing. On this particular morning, that thing is a wrong number, a phone call from someone looking for "Kevin." As he takes a bus to the Home Depot to buy wood glue for his mother, Jeff spots a guy wearing a "Kevin" sports jersey. At that point, of course, all bets are off. Jeff jumps off the bus to follow Kevin, and his adventure begins.
Meanwhile, Pat has sprung that new Porsche on his exasperated wife (Judy Greer), and her less-than-enthusiastic reaction sends them both off in directions they'll come to regret before the day is out.