Spoilers Please

Here's the synopsis as I recall it. I watched a few days ago so I might have missed something, but these are the essentials:

David and Dawn are a recently married couple who move back to David's town in the country from London, and are trying for a baby (even going to a fertility specialist for help).

They have bought an old farmhouse and are in the process of renovating it, when David's brother Nick appears unexpectedly. The brothers greet each other with fierce intense embraces, so Dawn later questions why she's never met him before. David says he's been away in the army at war, and when she pushes further he says that he pushed their mum down the stairs when he was 16, implying that he's violent and may have other issues as well.

Talk at the local pub with old, local friends about Nick's return reveals to Dawn that David and Nick come from a very bad home, and that David was beaten really badly by his father. Later in bed, Dawn questions David why he never said anything, but she doesn't get very far.

Dawn feels really compassionate towards Nick for what he's been through in combat, and they continue to get closer, and she also tries to make her life in the new town and joins a churchchoir, with one of David's old buddies, Gary who is married to the woman (Sinead) that Nick has always coveted, since school apparently. He even takes her to his and David's childhood home, which is abandoned and totally dilapidated.

The tension in the house escalates through Nick's PTSD and sleepwalking episodes, and David starts to become more tense and prone to outbursts himself. Dawn is really shaken by seeing this side of him, and comments that maybe she doesn't know him all that well at all (they met when she was mugged and he intervened and got married rather quickly). At one point the hen hutch door is left open and the dog kills the chickens, which Dawn is extremely attached to. David accuses Nick of doing it in one of his sleep walking bouts, which Nick ardently denies, and it comes out that it's actually DAVID who pushed his mother down the stairs. David later says it was to protect Nick so that his mother couldn't hurt him anymore, but it does show Dawn that the man she'd previously "never seen fight except when he saved me from the muggers" (and earlier comment), is actually much more prone to violence than she'd thought. At the end of that conversation, David tells her he just wants her to be happy.

Meanwhile, Gary starts pursuing her but she firmly deflects this, though sympathetically because she had earlier seen Nick having sex with Sinead. However she still continues to have a close, tender relationship with Nick, who apparently adores her. At one point Nick also leans in to kiss her but she firmly says no and then they hug. There are multiple shots of David watching these sort of scenes with an inscrutable expression on his face.

Then, when things are really bad at home between the brothers, David and Dawn have an appointment with the fertility specialist again, and it turns out that it seems David is infertile. The doctor offers to retest the sample, saying it might have just been a blip, but David admits that it's not "a blip;" he knew he was unlikely to have kids. He reveals that he had a former girlfriend who was trying to get pregnant and he'd found out then.

Dawn is furious, hurt, and extremely disappointed, and they get into a huge row on the car ride home, so that she ends up walking back. When she arrives home, Larry drops by, and they end up having sex (it's implied that she does it because she's so hurt and angry with David, although the scene cuts directly to it and it does slightly look like rape, it's a bit ambiguous. . .)

When David gets home they tightly and tearfully embrace, and discuss what to do with Nick. That night, Dawn wakes up to overhear an extremely heated and intense argument between the brothers in the guest room, with David throwing things and knocking things off the dresser in rage.

They attend a garden party the next day, where there are lots of babies and David sees Dawn holding a baby and he looks angst-ridden. Then towards the end of the party, lots of things comes out. Larry whispers nastily to Dawn at one point, "Who are you going to *beep* next?" Nick reveals that he had sex with Sinead, and David and Nick get into it and separately charge off into the woods.

When Dawn gets home, Nick is gone. She asks David what happened but he says he doesn't know, and they file a Missing Person police report. The next day, on a hunch, Dawn goes to their old family home and finds Nick living out of his car. She takes the chance to ask him about their childhood and he says that David really protected him and watched out for him, but that it means he sort of "owned" Nick. It's not really certain what he means by that.

But when she goes back again with David, Nick is gone, and she immediately suspects that David had something to do with it (although, again, it's not clear whether she thinks he forced him to move on, or he killed him or something...) Upset, she goes off on her own, but collapses to the ground in pain.

In the hospital David is by her side, and tells her that he "wants this, wants this child." So it turns out she's pregnant, and it's implied that David thinks it's Nick's child, which he seems to accept. Perhaps because (a) he wanted her to be happy and he thought that a baby was the answer, and (b) it was via someone he 'owned' in a sense, and maybe someone he also owed, as well (it's complicated). (Or if you want to be more cynical, it's because he killed Nick and his guilty conscience means he wants to take responsibility for his child).

But then about a year and a half later, David and Dawn are walking down the park lane with the baby, and they seem to really happy, and David is very loving to and doting of the baby boy. Then they run into Gary and Sinead, and as Gary takes the baby and holds it, realization dawns on David's face. The baby and Gary look identical and it's very obvious when they're side by side.

David looks at Dawn in sort of a different, calculating way at that point, but it's ambiguous whether it's enough to break the marriage because after a moment he resumes smiling and playing with the baby.

As for Nicks's actual fate... it's never revealed.