The musical Blood Brothers begins as its Narrator tells the audience about the Johnstone twins, Mickey and Edward, who were separated at birth and died on the same day. We next meet the twins’ mother, Mrs. Johnstone, a lower class woman who was abandoned by her husband after giving birth to five children, and while pregnant with another. She reminisces about the days when she used to go dancing with her husband, who made her feel like Marilyn Monroe. Now, however, her life is a never-ending cycle of unpaid bills and hungry children. She works at the house of Mrs. Lyons, a wealthy woman who longs for a child of her own.
Mrs. Johnstone is devastated to find that she’s carrying twins. There’s no way she can afford to feed two more mouths. An unlikely solution presents itself, however, in the form of Mrs. Lyons, who pleads to take one of the twins—but only if Mrs. Johnstone swears, on the Bible, never to reveal the truth of their bargain. The Narrator warns that misfortunes will follow. Soon after, Mrs. Johnstone gives birth, and as Mrs. Lyons takes one of her twin boys away, the poorer woman laments all the debts she’s had to pay. When she goes home, she lies to her children, telling them that one of the twins has died.
After Mrs. Johnstone returns to work, Mrs. Lyons grows jealous and suspicious, believing that Mrs. Johnstone is paying too much attention to the new baby. She proceeds to fire Mrs. Johnstone—and when the cleaning lady tries to take her baby back, Mrs. Lyons, knowing Mrs. Johnstone to be superstitious, comes up with a fatal lie. She tells Mrs. Johnstone that if two twins, separated at birth, ever learn the truth about their origins, they will die on the spot. Horrified, Mrs. Johnstone agrees to keep their secret. The Narrator warns that one day the Devil will come to punish the two women.
Seven years pass, and Mickey, the twin who stayed with Mrs. Johnstone, grows up in a rough-and-tumble environment. Edward, who grew up believing Mrs. Lyons to be his mother, matures in the lap of luxury. When still boys, the two meet by chance, and become fast friends. When they find that they share a birthday, they agree to become “blood brothers,” allying against Mickey’s bullying older brother, Sammy. When Mrs. Johnstone realizes that the two have met, she is horrified, and sends Edward away. Mrs. Lyons reacts even more violently, and contemplates uprooting her entire family in order to escape.
Despite their mothers’ disapproval, Mickey and Edward continue to see each other, and we witness a series of children’s games (many involving guns), as the two boys play with their other friend, Linda. The trio gets up to various pranks, eventually drawing the attention of the police, who threaten Mrs. Johnstone while flattering Mr. Lyons. Mrs. Lyons takes this moment to move her family to the country, despite Edward’s lack of enthusiasm. Before Edward leaves, however, Mrs. Johnstone gives him a locket with a picture of herself and Mickey, so that he can always remember them. The boys are lonely without each other, but the first act ends on an optimistic note: Mrs. Johnstone’s family is being relocated to the country as well, a move that she hopes will remove her children from a life of crime and squalor, and will help her to forget the sins of her past.
As Act Two opens, seven years have passed, and the boys are now fourteen. Both have become interested in girls, but feel awkward and unsure. Mickey and Linda, meanwhile, clearly have romantic feelings for each other, but Mickey’s lack of confidence has thus far kept them from any real connection. A moment of violence ruins this relatively calm beginning, as Sammy, now a full-fledged juvenile delinquent, attempts to rob a bus.
Mickey and Edward both struggle at school, with Mickey insulting a teacher, and Edward refusing to take off the locket despite his posh boarding school’s dress code. When Mrs. Lyons learns of his disobedience, she’s appalled, and she becomes even more upset when she sees the contents of the locket. The Narrator returns once again to remind Mrs. Lyons, and us, that the devil will be coming eventually.
After a failed romantic interaction with Linda, Mickey spots Edward, wishing that he could be suave and cool like “that guy.” Edward, meanwhile, longs for what he sees as Mickey’s freedom. The two meet, and after a moment, joyfully recognize each other. The two decide to see a porn film together, and set off for Mrs. Johnstone’s house together so that Mickey can get money—unaware that Mrs. Lyons is following them. Mrs. Johnstone is shocked but delighted to see her long-lost son. After the boys exit, Mrs. Lyons emerges. She accuses Mrs. Johnstone of stealing Edward’s affection, and claims that her son was never hers. She becomes violent, and attacks Mrs. Johnstone with a kitchen knife. Although eventually disarmed, she curses Mrs. Johnstone, calling her a witch, before exiting.
The boys meet up with Linda and experience yet another scrape with the police, before deciding to spend the summer together. An idyllic sequence follows, in which the trio transitions from fourteen to eighteen, glorying in the joys of youth and summer, even as the Narrator warns that soon, both their joy and their childhood will end. At eighteen, Edward—who has developed feelings for Linda—is going to university, while Mickey is working in a factory. With some encouragement from the self-sacrificing Edward, Mickey asks Linda to be his girlfriend, and she enthusiastically accepts.
In October, Mickey gives Mrs. Johnstone news: Linda is pregnant, and the two will be getting married. Their wedding, however, coincides with a severe economic downturn, and Mickey is fired. By the time that Edward returns for the Christmas holiday, his friend is downtrodden and careworn. Mickey tells Edward that he is still a child, and doesn’t know anything about life, claiming that the idea of blood brothers was just “kid stuff.” A rejected Edward meets up with Linda and confesses his love to her, but leaves after finding that she has married Mickey and is pregnant.
Mickey, impoverished and desperate, agrees to participate in a burglary with Sammy. The crime goes awry, and Sammy murders someone; he and Mickey are sentenced to jail. Imprisoned, Mickey becomes depressed, and is prescribed addictive antidepressants. After he’s released, he continues taking the pills, despite the pleas of his mother and his wife. Eventually, a desperate Linda asks Edward, now a city councilman, for help finding an apartment and getting Mickey a job. After Mickey reacts with anger at her efforts, the devastated Linda seeks comfort with Edward, and begins an affair with him.
As the two carry on their affair, Mickey resolves to stop taking his pills, for Linda’s sake. He’s derailed when Mrs. Lyons—fully unhinged—reveals Linda and Edward’s affair. The enraged Mickey finds a gun and sets out to confront Edward, followed by a distraught Linda and Mrs. Johnstone. The Narrator warns that the devil has arrived. Finding Edward in the town hall, Mickey accuses him not simply of the affair, but of secretly fathering his child, which Edward denies. As Mickey continues to threaten Edward with the gun, Mrs. Johnstone bursts in and tells the young men the truth: that they are twins, separated at birth. This revelation completely unhinges Mickey, however, as he realizes that he could be the one living Edward’s life. As he gesticulates wildly with the gun, he accidentally shoots and kills his twin, and is immediately shot and killed by the police in turn. The play ends with this horrific and bloody tableau, as the Narrator wonders what really killed the twins: superstition, or the British class system?