As the movie opens, we see a doctor informing Donnelly (Gleeson) that his wife died at 3 o'clock in the morning. He brings the man to his wife's bedside to say his last goodbye, excusing himself presently by saying he's unusually busy: there'd been 2 cot deaths, and a woman shot so brutally by her son "she had no head left on her".
Donnelly spends a few last moments talking awkwardly to his wife, even though he doesn't know what to say. He did bring the photo of 'David', which turns out to be a white pet rabbit. When he mutters, "I don't know where you are now", we get a glimpse of the inner turmoils of faith that he's experiencing.
Donnelly goes home on the train. He sits opposite a chain-smoking kid (Conroy), who reveals himself to be a manic foul-mouth. In his forlorn state, Donnelly tolerates him, allowing himself to be engaged in some inane conversations about 'tall jockeys' and, later on, Tony Curtis having a kid in his old age ("or is it Rod Steiger?"). Less accepting of the kid's abrasive manners are a couple sitting across the aisle. They are obviously "down in the dumps", but the kid just blathers on mindlessly and callously. After some animated exchanges, the kid leaves to get something from the buffet cart. At this time, Donnelly inquires, and finds out that the couple had just lost their son – cot death.
The kid comes back after the couple had left their seats. Donnelly tells the kid of the baby's death. The kid seems surprised, and immediately asks, "Did they kill 'it'?" Donnelly explains that it was 'cot death', to which the kid says, "That's what they all say". He surmises instead that they must have "banged it on something". He expounds by wondering aloud why more parents don't kill their kids, like Marvin Gaye's dad – "I would if I had a kid", he continues. Kids are rotten, he reasons, offering himself as an example ("I'm a fucking rotten kid!"). Upon the couple's return, things heat up again. The two almost come to blows had Donnelly not stepped in.
Donnelly goes to the buffet cart to get a drink to drown his sorrow. His brusque exchange with the buffet cart kid (played by Gleeson's son) serves again to accentuate Donnelly's inner struggle. The man comes out to get tea and engages Donnelly in a brief conversation. When he asks whether the kid is retarded, Donnelly says he doesn't think so, on account of the kid knowing what dressage is.
With her husband gone, the woman is left alone with the kid, who quickly moves over to harass her as she clutches a picture of the dead baby. The kid says that the baby looked like "your man off the Bronski Beat" (as we learn later, Jimmy Somerville – "the gay man"), and laughed "No wonder you banged it on something", directly accusing the mother of killing her baby because he was "ugly". Aghast, the woman steps on the table to get away from her tormentor, only to trip, fall, and tear the photo. Even now, the kid is oblivious of what he's doing: "Don't blame that on me", as the fallen woman looks at the torn picture and gives him the evil eye. That must have been the last straw. Moments later, the kid, back in his own seat, was wondering if he might have gone "a bit overboard", when he is startled by a thump and a big smudge on the train window; the woman had thrown herself out of the train. He goes to investigate, to find an open gate, and the torn picture of the baby on the floor.
Donnelly and the husband come back. The husband asks where his wife is. The kid nonchalantly informs him that his wife jumped off the train. Thinking that the kid was joking, the man declares him indeed retarded, and sets out to look for his wife. He does not hear the kid say that if he'd cared to look, he'd quickly see she'd jumped ("she's dripping down the half of it"). When Donnelly turns to see the blood on the window, he pulls the emergency stop. The train stops, the police arrives, and the man eventually learns of the horrible truth of his wife's suicide. Donnelly and the kid are questioned by a policeman, who asks the kid at one point, "Do I know you from somewhere?" After the investigation ends, and as the train was pulling away, the investigating policeman sees the kid wildly waving goodbye, and comes to the realisation that the kid is someone they want. He orders the train stopped, "and tell the boys to get the guns out".
On their way again, the kid tries to get Donnelly to take his side "Admit it, fella – she was getting on your nerves too with her bawling". When he gets no sympathy, the kid blurts out that HIS mother got murdered last night, but he's not mourning. As if to reciprocate, Donnelly tells the kid that his wife just died as well. He begins to weep. Trying to comfort his new-found friend, the kid suggests that it might not be such a bad thing, seeing how "she's up with God now". Donnelly tells him he does not believe in God any more – "today was the last straw". As if to cheer him up, the kid again presses Donnelly with his "deadly" story of a cow with trapped wind. When Donnelly capitulates, the young man is delighted. He tells this 'true' story, which was so bizarre, that Donnelly cannot help but laugh. "Best day of me fucking life – that cow exploding," the kid concludes.
As he approaches his station, Donnelly gets up to leave. "Sorry to hear about your dead missus now", offers the kid; Donnelly replies, "Sorry to hear about your mom", which the kid shrugs off with "Ah – no loss".
As the train is pulling into the station, Donnelly notices armed cops everywhere. It's obvious now that the kid is responsible for that murder to which the doctor alluded in the beginning. A shootout between the kid and the cops ensues, which leaves the kid mortally wounded. His last words are regrets that he didn't even hit one policeman. The kid dies as Donnelly gently cradles him like a son he never had. He then takes one of the kid's six shooters and hides it in his coat.
Now at home, Donnelly is getting ready to kill himself. He looks in the gun: 2 bullets. Just when he is about to shoot himself, he hears scratching sounds from David, his wife's pet rabbit. He takes David in his lap, gently cradling and stroking it, telling the rabbit "I'll be following you shortly", before putting a bullet in the rabbit's head. Ready for his final act, Donnelly nonetheless fumbles with the gun, drops it, and accidentally discharges it of its last bullet. Stunned, Donnelly looks at the useless smoking six shooter, then at the bloody rabbit in his arms. He heaves heavy sighs, looks skyward and moans, "Oh Jesus! What a fuckin' day."