The novel is split into five "books", each covering a different period in the characters' lives.
Book One opens in 1912 and introduces 11-year-old Francie Nolan, who lives in the Williamsburg tenement neighborhood of Brooklyn with her 10-year-old brother Cornelius ("Neeley" for short) and their parents, Johnny and Katie. Francie relies on her imagination and her love of reading to provide a temporary escape from the poverty that defines her daily existence. The family subsists on Katie's wages from cleaning apartment buildings, pennies from the children's junk-selling and odd jobs, and Johnny's irregular earnings as a singing waiter. His alcoholism has made it difficult for him to hold a steady job, and he sees himself as a disappointment to his family as a result. Francie admires him because he is handsome, talented, emotional, and sentimental, like her. Katie has very little time for sentiment, since she is the breadwinner of the family who has forsaken fantasies and dreams for survival.
Book Two jumps back to 1900, with the meeting of Johnny and Katie, the teenage children of immigrants from Ireland and Austria, respectively. Although Johnny panics and begins drinking heavily when Katie becomes pregnant with first Francie and then Neeley, Katie resolves to give her children a better life than she has known, resolving to follow her mother's insistence that they receive a good education. Kate resents Francie because the baby is constantly ill, while Neeley is more robust. Kate makes a promise to herself that her daughter must never learn of her preference for Neeley. During the first seven years of their marriage, the Nolans are forced to move twice within Williamsburg, due to public disgraces caused first by Johnny's drunkenness and later by the children's Aunt Sissy's misguided efforts at babysitting them. The Nolans then arrive at the apartment introduced in Book One.
In Book Three, the Nolans settle into their new home, and the children (now seven and six) begin to attend the squalid, overcrowded public school next door. Francie enjoys learning, even in these dismal surroundings, and gets herself transferred to a better school in a different neighborhood with Johnny's support. Johnny's attempts to improve the children's minds fail, but Katie helps Francie grow as a person and saves her life by shooting a child-rapist/murderer who tries to attack Francie shortly before her 14th birthday. When Johnny learns that Katie is pregnant once again, he falls into a depression that leads to his death from alcoholism-induced pneumonia on Christmas Day 1915. Katie cashes in the children's life insurance policies and uses that money, along with their earnings from after-school jobs, to bury Johnny and keep the family afloat in 1916. The new baby, Annie Laurie, is born that May, and Francie graduates from grade school in June. Graduation allows Francie to finally come to terms with the reality of her father’s death.
At the start of Book Four, Francie and Neeley take jobs, since there is no money to send them to high school. Francie works first in an artificial-flower factory, then gets a better-paying job in a press clipping office after lying about her age. Although she wants to use her salary to start high school in the fall, Katie decides to send Neeley instead, reasoning that he will only continue learning if he is forced into it, while Francie will find a way to do it on her own. Once the United States enters World War I, in 1917, the clipping office rapidly declines and closes, leaving Francie out of a job. After she finds work as a teletype operator, she makes a new plan for her education, choosing to skip high school and take summer college-level courses. She passes with the help of Ben Blake, a friendly and determined high school student, but she fails the college's entrance exams. A brief encounter with Lee Rhynor, a soldier preparing to ship out to France, leads to heartbreak after he pretends to be in love with Francie, when he is in fact about to get married. In 1918, Katie accepts a marriage proposal from Michael McShane, a retired police officer who has long admired her and has meanwhile become a wealthy businessman and politician.
As Book Five begins in the fall of this same year, Francie, now almost 17, quits her teletype job. She is about to start classes at the University of Michigan, having passed the entrance exams with Ben's help, and is considering the possibility of a future relationship with him. The Nolans prepare for Katie's wedding and the move from their Brooklyn apartment to McShane's home. Francie pays one last visit to some of her favorite childhood places and reflects on all the people who have come and gone in her life. She is struck by how much of Johnny's character lives on in Neeley, who has become a talented jazz/ragtime piano player. Before she leaves the apartment, Francie notices the Tree of Heaven that has grown and re-sprouted in the building's yard despite all efforts to destroy it, seeing in it a metaphor for her family's ability to overcome adversity and thrive. In the habits of a neighborhood girl, Florry, Francie sees a version of her young self, sitting on the fire escape with a book and watching the young ladies of the neighborhood prepare for their dates. Francie says, "Hello, Francie", to Florry, and then, "Goodbye, Francie", softly, as she closes the window.
Mary Frances "Francie" Nolan is the protagonist. The novel begins when Francie is 11 years old. The rest of the novel tells of Francie's life until she goes to college at 17. Francie grows up in Brooklyn in the early twentieth century; her family is in constant poverty throughout most of the novel. Francie shares a great admiration for her father, Johnny Nolan, and wishes for an improved relationship with her mother, hardworking Katie Nolan, recognizing similar traits in her mother and herself that she believes are a barrier to true understanding. The story of Francie traces her individual desires, affections, and hostilities while growing up in an aggressive, individualistic, romantic, and ethnic family and neighborhood; more universally it represents the hopes of immigrants in the early twentieth century to rise above poverty through their children, whom they hope will receive "education" and take their place among true Americans. Francie is symbolized by the "Tree of Heaven" that flourishes under the most unlikely urban circumstances.
Katie Rommely Nolan is Francie's mother and the youngest of her parents' four daughters. She is a first-generation immigrant with an evil father and an angelic mother who emigrated from Austria. She married Johnny Nolan when she was only 17 years old. Katie is a hardworking, practical woman whose youthful romanticism has been replaced by a frigid realism that often prevents her from sympathizing with those who love her most. She runs her home in such a way that her children are able to enjoy their childhood despite their extreme poverty. Because Johnny is an alcoholic and can rarely hold down a job, Katie becomes the family breadwinner by cleaning apartment buildings. Johnny, however, is more attuned to Francie's hopes of graduating from high school and becoming a writer. As Francie matures and develops an inclination toward academia, Katie realizes she is more devoted to Neeley than to Francie. Katie becomes pregnant just before Johnny dies and survives on her own until she agrees to marry Sergeant Michael McShane, a pipe-smoking local policeman-turned-politician.
Sissy Rommely is Katie's oldest sister and one of Francie's three aunts. Because of her parents' immigration and lack of knowledge in their new environment, Sissy never goes to school and is therefore illiterate. Sissy is kind, compassionate and beautiful, and many men fall in love with her. She is first married at 14, but after being unable to have any live children with her husband, Sissy leaves him. She marries two more times without ever getting a divorce. In between marriages, Sissy has a number of lovers. She calls each of her husbands and lovers by the name "John" until her final husband, who insists that she properly divorce her second husband and demands to be called by his own name, Steve. Sissy has ten stillborn children, but adopts an immigrant girl's baby daughter born out of wedlock and eventually gives birth to a healthy son of her own.
Johnny Nolan is Francie's father. He is a first-generation American; his parents immigrated from Ireland. He has a protective mother and had three brothers, all of whom died young. Johnny marries Katie Rommely at nineteen. He is charismatic, a loving husband and father, loved dearly by his family but especially by Francie. He is, however, an alcoholic. When he does hold a job, Johnny works as a singing waiter. He has a beautiful voice, a talent that is greatly admired but that is largely wasted because of his reputation as an alcoholic. After Katie tells him that she is pregnant with their third child, he stops drinking and immediately falls into a deep depression that ends with his death from alcoholism-induced pneumonia. He is a dreamer, in sharp contrast to Katie, whose view of the world is realistic.
Cornelius "Neeley" Nolan is Francie's little brother. He is a year younger than Francie and is favored by his mother, Katie. Neeley is an outgoing child who is more widely accepted by the neighborhood children than Francie. He shows more emotion when his father dies than Francie, who reacts to the loss by becoming even more determined to get an education and rise above her mother's limited vision. Neeley refuses to follow the tradition of Nolan men and determines to never become an alcoholic. Like Francie, he feels that their childhood was pleasant despite their poverty.
Eva "Evy" Rommely Flittman is Katie's youngest sister and Francie's other aunt, playing a role more minor than Sissy's. While considered throughout most of the novel to be in less dire circumstances than Katie, Evy struggles with her lazy husband Willie, a milk-wagon driver. When Willie suffers an injury, Evy drives the route instead and proves surprisingly good at it, treating the horses much more kindly than Willie does. At the end of the novel, he leaves her to travel as a one-man band and she carries on without a husband. When McShane gives Katie $1,000 as a wedding present, she passes $200 on to Evy - the value of Willie's life insurance policy. Unlike Sissy, Evy has had only one marriage and is not assumed to be promiscuous. She has three children, a girl (Blossom), and two boys (Paul Jones and Willie, Jr.).
Eliza Rommely is Francie's third aunt that is only mentioned once. She became a nun because of her mother's love of the Catholic church. Francie only met her once and Eliza was the reason Francie didn't want to become a nun for her facial hair.
Thomas and Mary Rommely are the parents of Sissy, Eliza, Evy, and Katie; they emigrate to America from Austria just before Sissy is born. While Thomas hates America, enjoys tormenting Mary, and forbids the speaking of English at home, Mary patiently endures her hardships and serves as a moral/practical guide for her daughters. Mary cannot read or write English, but she encourages Katie to ensure that her children learn the language, and also to begin saving money so she can buy land someday. The Rommelys' second oldest daughter, Eliza, is mentioned only briefly; she becomes a nun and joins a convent.
Flossie Gaddis is one of the Nolans' neighbors, a single woman who scares men away as she constantly looks for new relationships. She keeps her right arm covered at all times to hide scars from a childhood accident with a tub of scalding water. She has a brother, Henny, who is dying of tuberculosis.
Lee Rhynor is Francie's first love, a soldier on leave who tries to manipulate Francie into sleeping with him after he wins over her heart. When Francie refuses, he goes back to his fiancee.
Ben Blake is a boy Francie befriends during her first summer of college classes. Ben is driven and determined. While he is the object of Francie's affection at first, she feels differently after falling in love with Lee. However, at the end of the novel, Francie goes to college with a promise ring from Ben and hope of a future with him.
Although the book addresses many different issues—poverty, alcoholism, lying, etc.—its main theme is the need for tenacity: the determination to rise above difficult circumstances. Although there are naturalistic elements in the book, it is not fundamentally naturalistic. The Nolans are financially restricted by poverty yet find ways to enjoy life and satisfy their needs and wants. For example, Francie can become intoxicated just by looking at flowers. Like the Tree of Heaven, Brooklyn's inhabitants fight for the sun and air necessary to their survival.
Idealism and pragmatism are weighed and both found necessary to survival in Brooklyn. Johnny lies about his family's address in order to enable Francie to attend a better school, presenting Francie with opportunities that might not have been available to her otherwise. Sissy helps Johnny recover from alcoholic withdrawals by appealing to his libido, helping Katie and Johnny to stay together despite Johnny's disease. Katie explains love and sexuality to Francie from two somewhat clashing points of view: as a mother and as a woman. The book revises traditional notions of right and wrong and suggests pointedly that extreme poverty changes the criteria on which such notions, and those who embrace them, should be judged.
Gender roles are more fluid in A Tree than in previous novels about young people. Katie's hands grow rough as she performs physical labor while Johnny's hands remain smooth and he wears expensive clothing. Francie doesn't fully begin to realize her own femininity until she can prove useful to her mother in childbirth. As Francie discovers her desire for companionship, she begins to understand the injustices women are often forced to endure when pregnant out of wedlock.
Other issues the book addresses include:Man vs. his environment
Coming of age/loss of innocence
The American Dream
Human Life/ What it means to be human
The book was adapted into a 1945 film directed by Elia Kazan, starring James Dunn, Dorothy McGuire, Joan Blondell, and Peggy Ann Garner, who won a Special Academy Award for Outstanding Child Actress of 1945. James Dunn won an Academy Award for Best Supporting Actor in the film.
George Abbott collaborated with the book's author to produce and direct the story as a 1951 Broadway musical, with music by Arthur Schwartz and choreography by Herbert Ross. The show starred Shirley Booth, Marcia van Dyke, and 12-year-old Nomi Mitty (as Francie). It ran for 267 performances.
A 1974 "made for television" movie adaptation, based on the film's screenplay, stars Cliff Robertson and Pamelyn Ferdin. This TV movie was a pilot for a weekly series drama which was never produced.