'What if I lose it and I never get it back again?" mumbles the mop-topped Beach Boy Brian Wilson (Paul Dano - just about brilliant) at the start of Love & Mercy. Alone in a recording studio, his whisper melts into white noise - tremulous, filled with dread.

A breathtaking psychological biopic, Love & Mercy toggles back and forth between the mid-'60s heyday of the California band and the 1980s, when the songwriting savant Wilson, suffering from auditory hallucinations and a shattered psyche, finds himself under the despotic care of therapist - and legal guardian - Eugene Landy. Paul Giamatti plays the latter with a fertile toupée and fevered glare; look up control freak in the dictionary and there should be a picture of Giamatti's evil doctor accompanying the definition.

Acting - and quaking in grim disrepair - opposite Giamatti in these fast-forward sections of the film is John Cusack. The actor has a difficult job here: Following Dano's uncanny realization of the younger Brian is tough enough, but Cusack has to project both the fragile, fragmented mindset of a man wrongly (and over-) medicated and the pop maestro, the sad, dreamy romantic who still lives somewhere in Brian's soul.

Elizabeth Banks, overcoming a wardrobe of terrifying Dynasty-era dresses and accessories, is great as Melinda Ledbetter, the Cadillac saleswoman who becomes involved with Brian, despite Landy's misgivings, and whose presence in the middle-aged Beach Boy's life offers the hope of redemption.

There are things about director Bill Pohlad's brave undertaking that don't quite work, such as a third-act montage of Brian in every iteration (including as a boy), trapped in his bedroom in catatonic reverie.

Pohlad goes all Kubrickian, at once too arty and too obvious. But the restaged 1960s promo films, the reenacted tour and concert footage and, most significantly, the recording sessions for what would become the landmark album Pet Sounds are all pulled off flawlessly and inventively. The young Dennis Wilson (Kenny Wormald), Carl Wilson (Brett Davern), Al Jardine (Graham Rogers) and Mike Love (Jake Abel) are perfectly cast, coiffed and costumed.

Mike Love, the Wilson brothers' cousin, shared songwriting credits with Brian Wilson on some of the Beach Boys' biggest hits, but as Brian began to expand the musical possibilities (kettle drums! bicycle bells! bobby pins on piano strings!) and expand his mind with psychedelics, the two collaborators were at odds.

A swimming pool scene, with the members of the band (and newcomer Van Dyke Parks, played by Max Schneider) virulently debating the creative turns Brian has taken, is pure, whimsical genius. Brian, barely hanging onto his float and his sanity, is at the deep end. He pleads for Mike and the others, at the opposite end, to join him so they don't have to yell. "We're too shallow for the deep end," Mike replies, saying it all.

Love & Mercy, with a script from Oren Moverman and Michael A. Lerner and the blessings of the Wilson and Ledbetter, may not be historically accurate on every count. But its daring dive into the mind of Brian Wilson feels right. God only knows (to borrow a Pet Sound song title or two), but you still believe in . . . Brian.