The series follows fraternal twins Kate and Kevin along with their adopted brother Randall as their lives intertwine. Kate and Kevin were originally part of a triplet pregnancy, conceived in a bar bathroom during Super Bowl XIV; however, their biological brother was stillborn. Their parents, Jack and Rebecca, intent on bringing home three babies, decide to adopt another newborn (Randall), a black child who was born on the same day, and brought to the same hospital after his biological father abandoned him at a fire station.
Episodes weave through the stories of the past and present of the characters. The flashback episodes take place in Pittsburgh, while the current time episodes are split between Los Angeles, New Jersey, and New York City.
The intertitle references a Wikipedia-sourced statistic about how many people share the same birthdays. Jack Pearson is celebrating his 36th birthday when his wife Rebecca goes into labor with their triplets. Personal assistant Kate is celebrating her 36th birthday by recommitting to lose weight and befriends Toby at counseling. Her brother Kevin, famous for starring on the sitcom The Manny, is celebrating his 36th birthday by acknowledging his dissatisfaction with his role and abruptly quitting in front of a live audience. Successful businessman Randall is celebrating his 36th birthday by tracking down the father who abandoned him at a fire station on the day he was born. Rebecca loses one of the triplets during birth. It is revealed that Kate Pearson and Kevin Pearson are the surviving pair, while Randall Pearson – brought to the same hospital by a fireman – is their adoptive brother. Jack and Rebecca's story takes place in 1980.
Rebecca and Jack are having marital issues. Randall's wife, Beth, has suspicions about his biological father's motives. Toby and Kate start getting closer. Kevin is contractually obligated to stay on his sitcom for two more years. Kevin tries to turn to Randall for advice, but it is revealed that they have had a strained relationship since childhood. In the present day, Rebecca is remarried to Jack's best friend, Miguel.
Randall's name was initially going to be Kyle. His biological father William (nicknamed Shakespeare) met his birth mother on a bus ride where they bonded over poetry, but it's clear that at some point they also developed substance abuse problems. William is shown on the bus as he takes his newborn son to the fire station. Rebecca is not able to bond well with "Kyle", and Jack eventually admits he cannot either. Rebecca prevents William from seeing his son, but renames him Randall after William's favorite poet, Dudley Randall. Randall's daughters still do not know who William really is but, after Rebecca scolds him at Randall's home, he leaves – Randall then scolds him even harder. Kevin is serious about moving to New York and wants to bring Kate with him, who is shown to have a talent for singing, but no desire to showcase it. Toby makes grand gestures for Kate, but feels like she gives precedence to Kevin. William's cancer is terminal. Kevin fires Kate out of love. Toby and Kate finally have sex.
The Pearsons go to the pool, but have set-backs: Kevin almost drowns while his parents attend to Randall and Kate; Kate is ostracized for being overweight by her classmates; and while Randall befriends other Black children, Rebecca takes it especially hard when one mother suggests some grooming tips for her Black son. While Kevin is determined to land a serious role on Broadway, even his nieces will not let him forget The Manny. William is wrongly accused of loitering in Randall's nice neighborhood and is annoyed when Randall does not defend him. Toby runs into his ex-wife while on a date with Kate. Kevin royally bombs his audition, but his Tony-nominated scene partner begrudgingly tells him he got the role because of his The Manny fame; in a flashback, Kevin is shown to have been insecure since childhood, which may have led him to acting. Kate gets a job from Toby's ex-wife Josie, who Toby professes is not the angel she seems to be; in fact, her affairs led to his weight gain and caused him to contemplate suicide. Kevin temporarily moves in with Randall.
The Pittsburgh Steelers are winning the Super Bowl XIV, but Jack and Rebecca are fighting about not having kids; Kevin and Kate are conceived that night. Miguel and his wife are present at the time. Beth Pearson believes she may be pregnant again, but it turns out she is not. William reveals to Kevin he is a big fan of The Manny and helps Kevin realize he should stop doubting himself. Kevin believes he moved to New York out of fear and uses the play to teach a life lesson to his nieces; he also has a penchant for painting like Jackson Pollock. Kate acts weird about watching a Steelers game and has to tell Toby why football games are personal for her. It is then confirmed that Jack died at some point.
Randall trades commodities for businesses based on weather futures, a job no one seems to understand, and he tries unsuccessfully to explain that on Career Day. Flashbacks reveal that Randall has been extraordinarily intelligent since he was a child, which got him transferred to an elite private school. Kate starts a new job as a personal assistant for Marin (Jami Gertz) and has to deal with Marin's bratty teenage daughter, Jemma. Jemma, who is overweight, is sure that her mom hired Kate over the other candidates as a way to get through to her. Kate reveals she no longer talks to her mom, Rebecca. Olivia has unorthodox methods of teaching Kevin how to deal with grief. Randall decides he wants to take piano lessons, but not from William.
The Pearson children are teenagers by now in the flashback and Kevin and Randall are bickering harder than ever. Kevin moves out of their room and into the basement and wants Randall to stay away. In the present, Kevin and Randall go out to dinner by themselves when Rebecca and Miguel have to cancel. It appears Kevin purposely sat them at a large table with other people so that they would not have to be alone, but he abruptly leaves when he finds out Randall has never actually seen The Manny. Kate is working hard on trying to lose weight, but is disappointed with the results; meanwhile, Toby is hitting his weight-loss goals without trying and wants to abandon the diet altogether. Beth and William smoke marijuana together and he accidentally reveals to her that he has known Rebecca since Randall was a baby. Ironically, Beth moves Kevin's stuff to the basement.
In the past, the Pearsons, as children, are going on a 6-hour road trip to Rebecca's parents' house for Thanksgiving, but they get into a car accident and abandon the dinner completely. Kate takes a break with Toby before Thanksgiving, saying she needs to get a handle on her weight and her life. Kevin invites Olivia to come celebrate Thanksgiving at Randall's house. Beth gives Rebecca an ultimatum, since she previously had 36 years to tell Randall the truth, but he finds out before Rebecca can tell him. William advises Olivia be kinder to Kevin while she has the chance. Kevin allows Miguel to participate in a Pearson Thanksgiving tradition. After a disastrous flight, Kate announces a life-changing decision.
Randall is unbearably devastated by his mother's lies and betrayal. He, Kate, and Kevin make a trip to their family cabin; Olivia tags along with her uninvited ex-boyfriend Asher and playwright Sloane. Olivia makes a rude comment to Kate, driving a wedge between her and Kevin. Child Randall thinks he can find his birth parents by the genetic trait of tongue rolling. Rebecca has never told Jack that she knows who his real parents are and she never will. Adult Randall has a vision of Jack at the cabin. Though "Vision Jack" is shocked Rebecca lied to him too, he tells Randall to understand his mother's point of view. Kate still wants to be friends with Toby, but he would rather not. Kevin does not like how Olivia and Asher are behaving and kicks them out, ending his relationship with her. He later ends up sleeping with Sloane. Randall tells his mother he will not speak to her again until Christmas.
When young Kate comes down with appendicitis on Christmas Eve, the Pearsons take her to the hospital, where they run into Dr. K, whom they realize needs their support as he is dying. Kevin celebrates Hanukkah with Sloane's family and convinces her to star in her own play when he sees she has acting talent. Kate consults a doctor with her mother about the life-changing surgery she wants to have. Randall talks his co-worker out of a suicide attempt. William is revealed to be bisexual. Toby comes to Randall's house to surprise Kate, and they get back together, but he later collapses and is sent to the hospital and operated on.
It is revealed that Toby has cardiac arrhythmia instead of a heart attack. Jack and Rebecca are looking for a house that could fit their growing family, but did not expect to be having triplets. Randall feels William's time spent with Jesse is taking precious moments away from their family but is assured it is only to help him with his medical needs. Kevin and Sloane are doing well--until Olivia comes back. Jack asks his estranged, abusive father for money and builds a bigger house. Kate and Toby discuss marriage.
The episode is completely based around the lead-up to the triplets' birth. Rebecca is going through a mental breakdown, which includes forgetting it is Jack's birthday. Miguel tries to encourage Jack to celebrate one more birthday before he becomes a father by taking him golfing. Dr. K struggles to accept the death of his wife, who died 14 months earlier, including refusing to dispose of her belongings, and also declining an offer for dinner at a widow's house. Fireman Joe is struggling with his marriage, and he asks a priest for a miracle. William drops off Randall as a newborn at the firehouse. Joe decides to take it first to his wife, who refuses to look after him even though they could not conceive. Joe takes the baby to the hospital and Jack decides to adopt him following of the death of his third triplet. The next day, Dr. K decides to throw away his wife's belongings and eat dinner at his friend's house.
The triplets' tenth birthday is approaching, and instead of keeping of the family tradition of celebrating the day, Kevin and Kate encourage their parents to throw them separate parties, in which each of the triplets has to invite their entire class. At the birthday party, Randall only has a handful of classmates show up while Kevin and Kate have full parties. When Kate's best friend Sophie arrives, she and Kate's entire party merge into Kevin's party, leaving Kate alone, in which Jack comes to comfort her. In the current day, Randall is trying to bid for a project in Calgary, but is interrupted by William, who recently stopped chemo treatments and is now energetic. Kate's doctor encourages her to attend a camp instead of having weight loss surgery. After Kevin breaks up with Sloane and Toby drops Kate off at the camp, the two go to a bar together. Following Toby's advice to pursue the woman he really loves, Kevin visits and tries to reconcile with Sophie, his childhood love and ex-wife. A mysterious worker at the camp tries to talk to Kate, who is now engaged to Toby, and acts like he is trying to seduce her.
Miguel and Shelly get divorced and fearing their marriage will lead to the same fate, Jack tries to bring excitement to his and Rebecca's relationship. Kevin tries to reconcile with his ex-wife Sophie. Toby surprises Kate at the camp she is attending and is met with unkindness. Stress mounts and Randall cannot cope. Randall misses a business meeting he planned fully knowing his daughter's chess tournaments was on the same night.
Kate gets kicked out of fat camp and apologizes to Toby. They begin asking questions about each others' lives to strengthen their relationship, though Kate cannot answer when asked about how her father died. Rebecca wants to go on tour with her band, but after finding out she dated a member of the band, Jack does not want her to go. They have a fight and Jack has his first drink in years. Kevin is about to open his play when Randall calls him and is distressed. Kevin has a flashback to when Randall used to have nervous breakdowns over self-inflicted pressure. Kevin leaves the theater to find and comfort him.
Randall drives William to Memphis, his hometown. On the trip, the two stop at a park where some of Jack's ashes were spread by a tree; William pays his respects and thanks Jack for raising Randall. Intermittently, William's relationship with his mother while growing up is shown, where she leaves him as a young adult to take care of relatives in Pittsburgh. William at the time is playing in a cover band in his cousin's club, where he begins to write music to grow the band until he learns his mother is sick and decides to move to Pittsburgh, where she dies. In the present day, William returns to his old home and takes out his buried treasure (a couple toys and quarters). He then takes Randall to his cousin's club, who is still angry at him for leaving the band when he did, but forgives him and lets him play. The next morning, William is unconscious in his bed and is transported to the hospital, where doctors tell Randall that he has hours to live. William wakes up to have Randall retrieve his "Book of Poems for My Son" before he admits he's glad that the two most important people in his life (his mother and his son) were there at the beginning and end of his life, respectively. Randall then uses Jack's calming down mechanism to help ease William's fear of dying. William passes.
Rebecca leaves for her tour. Kate encourages Jack to attend Rebecca's first show; after rejecting a colleague's advances at a happy hour, he sets out driving to the show despite having been drinking. Per William's wishes, Tess and Annie plan a celebration of his life, while Beth feels left out; Randall asks her to toast William, and she receives a loving postcard William sent from Memphis. Rebecca admits she kept William a secret because she feared losing Randall; Randall acknowledges that he and William had the time they needed, and he and Rebecca reaffirm their love. Kevin's play premieres and the family loves it, though an important critic fails to attend. Randall quits his job over the firm's disrespect and lack of appreciation, admitting he doesn't have a plan. Kevin and Sophie consummate their relationship, and Ron Howard offers Kevin a film role after seeing the play. Kate struggles to tell Toby about Jack's death, saying she is responsible for it.
Jack drives while drunk to Cleveland to see Rebecca's show, where her ex-boyfriend Ben tries to kiss her moments before the band is supposed to go on. Jack finds out and punches him, prompting Rebecca to drive them home. In flashbacks, Jack and Rebecca are shown in 1972, where they are both looking for work and set up on blind dates. Jack and a friend head to a poker match, with Jack's life savings, and win big on the first game, but are then are robbed by the other players outside. When Jack and Rebecca return home, they fight over Jack's relapse and over Rebecca feeling empty, adding that Jack does not love her for who she currently is. Back in 1972, Rebecca goes on a blind date with a successful businessman, but leaves early so she could sing at an open mic night. Jack devises a plan to steal money from a register at a bar, but is distracted when he sees Rebecca singing. Jack makes up for the argument by admitting what he loves about her, and while he sets out to spend some time at Miguel's, he says that their love story "is just getting started." Intertwined with Jack's apology, Kate and Toby are shown returning to Los Angeles and after seeing a photo of Rebecca, Kate decides she wants to sing. Kevin says good-bye to Sophie as he heads off to meet with Ron Howard about a film role. After Randall looks through a photo album of his younger self and his family, he admits to Beth that he wants to adopt a baby.