“Brothers” is a movie built on that jarring disconnect between combat zone and “back home.”

Part POW thriller, part romance, with a big helping of melodrama, Jim Sheridan’s film is about a brother who went off to war, was declared dead, but returns a changed man. And it’s about the brother left behind, a man changed by his soldier sibling’s sacrifice, and by stepping into his brother’s role with the missing man’s family.

A mature, lean Tobey Maguire is Capt. Sam Cahill, the son who followed Dad (Sam Shepard) into the Marine Corps and is about to go back to Afghanistan. Jake Gyllenhaal is perfectly cast as Tommy, the prodigal son we meet as Sam picks him up from prison. Tommy always drank too much and got into trouble. Sam is the alpha male the old man is proud of.

“Why don’t you try mimicking your brother for a change?” is Dad’s only advice, picking up their father-son fight the moment the convict gets home.

Sam’s wife, Grace (Natalie Portman), has hated the slacker Tommy since high school. When Sam ships out and his helicopter goes down and he is listed as dead, that doesn’t change.

Adapting a Danish film by Susanne Bier, Sheridan (“My Left Foot”) gives away Sam’s fate – he’s a prisoner of the Taliban – right away. He contrasts Sam’s ordeal with Tommy’s transformation back home. Sam is tortured while Tommy charms one and all to pop-music accompaniment. Sam is tested, and Tommy starts to think about someone other than himself – caring for his brother’s young daughters and widow.

And then Sam comes home.

The home-front scenes are so cute – skating dates, making pancakes for Mommy, cute contractors helping Tommy fix up Grace’s kitchen – that they feel like another movie set in another world. But that’s how vets describe their dislocation trying to talk with people who “wouldn’t understand.”

Sheridan, at home working with kids and dealing with dark subjects, doesn’t quite get the balance right, especially when you compare “Brothers” with the superior soldiers’ homecoming drama “The Messenger,” which has more weight, less melodrama and rarely hits “cute.”

But as predicable and cloying as “Brothers” sometimes is, the cast is fascinating – young actors now possessing the dramatic heft to pull something like this off. Maguire, as Sam, comes unhinged in subtle, realistic ways and Gyllenhaal and Portman react to him with a convincing blend of fear and pity. Putting them all at a table with Shepard and veteran character actress Mare Winningham makes for intimate, beautifully played drama, even if we have a feeling that we’ve seen all this before.