IN "A HOME at the End of the World," people come and go through Bobby and Jonathan's lives: family members, neighbors, short-term lovers and one newborn. But Bobby and Jonathan -- friends, onetime lovers and virtually brothers -- are rarely apart. They're family in the oddest way.

Michael Mayer, a Washington native, has made an emotionally compelling movie about two men's evolving relationship. The film's first half is easily the best and brightest. As the movie moves into the more saddening sections, however, it loses most of its power.

The result is a streamlined account of these boyhood friends from 1967 through the early 1980s, as they grow into adolescents, then form an unusual triangle with Clare (Robin Wright Penn), a free-spirited eccentric who dreams of a conventional home and a baby. During this extended span of time, the world evolves from the free love of the Woodstock era to the sexually chilling age of AIDS.

The story opens with a tragic event that takes place in the heyday of the dope-smoking Sixties. This episode, as well as the deaths of other family members, leaves 15-year-old Bobby Morrow (Erik Smith) homeless. So when Jonathan Glover's family informally adopts Bobby, an elfin naif who took his first acid trip at age 9 (played in his early years by Andrew Chalmers), he's only too willing to join.

Bobby is quite the human tonic. He introduces Jonathan's mother, Alice (Sissy Spacek), to marijuana, and helps Jonathan (Harris Allan) realize his sexual persuasion. Bobby is so easygoing, he reciprocates Jonathan's physical desires. He'll do anything, it seems, to stay in this new family and avoid being lonely. The problem is, Jonathan falls in love with Bobby for life. No matter what happens to them as men, his devotion and expectations will always be there.

Bobby realizes this soon enough in 1982, when he moves in with Jonathan and his roommate Clare. Although Jonathan is clearly gay and launches into regular jaunts with men, he has had some kind of history with Clare. She seems to be in love with him. Neglected and needy, Clare lures Bobby into a relationship. It turns out to be his first heterosexual encounter. Jonathan is not pleased.

And so ends the bright, major-chord section of the movie when everyone is getting to know each other. As the story moves into the minor-chord second and third acts, which mark the sobering of life, the tempering of life's mysteries with sad, dawning realizations, the movie loses emotional momentum.

The film's biggest problem is the amount of narrative ground it must traverse. Scriptwriter Michael ("The Hours") Cunningham, adapting his own novel, simply doesn't have the screen time to explore the main and subsidiary characters and all the interesting things they experience. "Home" becomes an elliptical, episodic closeout. The gaps in time between incidents are huge, and our emotional connection with the characters wanes.

What keeps the movie going, far longer than the screenplay deserves, are fine performances all around. As the unfathomable, adult Bobby, Colin Farrell exudes a tremulous, shy quality. It's like nothing he has shown us before. (As Bobby's younger versions, Smith and Chalmers are also strong.) Dallas Roberts has a wonderfully effortless quality as the seemingly affable but complex Jonathan. The women are top-notch, too. Spacek can mark another high watermark in her career. An early scene in which she gets to know the pleasures of cannabis is the movie's standout. And Wright Penn, one of our most underrated actors, fairly coruscates at times as the life-affirmative Clare, whose determination to make sense of her relationships with Bobby and Jonathan is the movie's secret ingredient.

A HOME AT THE END OF THE WORLD (R, 95 minutes) -- Contains drug use, sexual scenes and obscenity. At Cineplex Odeon Dupont Circle and Landmark's Bethesda Row.

Robin Wright Penn and Colin Farrell give stand-out performances in the drama "A Home at the End of the World."