As the Peasrons gather to celebrate their 40th birthday, Randall spends the birthday with his own family.
The birthday celebration continues through flashbacks from the day they were born. Kate and Madison have a big decision to make regarding their future, while Randall is not being honest.
In a flashback, the Big Three, as eighth graders, experience puberty. while in the present, Kate and Toby meet an important person regarding their adoption plan. Kevin and Madison struggle to connect.
Rebecca and Jack are overwhelmed by their crying babies. As Ellie tells Kate she considered an abortion and is confident about the adoption, Kate recalls her hidden secret. Randall offers Malik an internship.
In a flashback, Kate decides to get an abortion without telling anyone after visiting her dismissive and inconsiderate boyfriend, Marc. In the present, she visits Marc, as he is in San Diego, and confronts him. Elsewhere, Kevin and Randall make peace after the former's call for advice.
While Randall's past is revealed, Kevin is trying to get back to LA from Vancouver because Madison is in labor.
Many incidents occur as Kevin is trying to get back home for Madison as she is alone at the hospital.
As Madison gives birth to the twins, Ellie, too gives birth and wishes her baby a good life. Rebecca is thrilled as everyone greets the babies via video chat. Kate names the newborn cousins "the new Big Three."
After everyone is back home, each one starts a new chapter in his life.
As tensions flare at Randall's home, Toby's unsuccessful job interview sours the evening when Kate's family visits Kevin's for dinner.
When Nicky gifts the babies Grisham thrillers from the airport bookstore, Kevin finds a surprise.
Madison informs Kevin that she wants a smaller wedding compared to what he was planning. Kate begins her job as a teaching assistant. Randall attends his first transracial adoption support group meeting. Kevin wants to have a discussion with Randall after asking him to be his best man.
Kevin and Randall have their heart-to-heart discussion in Philadelphia.
After Beth's dance studio is shut down due to the COVID-19 pandemic, she is looking for a new career path. Meanwhile, Madison, Kate, and Rebecca go wedding dress shopping
Kevin and Madison's plans for their bachelor and bachelorette parties didn't go as well as they wished for.
As the Pearson family gathers for Kevin and Madison's wedding, flashbacks of Madison's life are featured.