David gets the money to pay off his debts, reveals his identity to his 500 biological children, and gets Emma to agree to marry him so he can raise their baby (and Emma also accepts their new baby's 500 siblings into her life).
David (Vince Vaughn) works as a delivery man for his family's meat company (Bobby Moynihan plays one of his brothers). He is $80,000 in debt to loan sharks after using the money to invest in various endeavors that turned out to be fraudulent (he is kindly but overly trusting). He also learns that his girlfriend Emma (Cobie Smulders) is pregnant but doesn't think David is reliable enough to raise their child with her.
David is later contacted by a sperm bank he donated to in his youth. It turns out that he donated almost 700 times and, as a result, is the biological father of over 500 children. The children have somehow discovered they are all related and are filing a suit against the sperm bank to reveal the name of their biological father (David and the recipient families signed a confidentiality agreement and David used an alias when he donated). David talks to his attorney best friend Brett (Chris Pratt) and Brett agrees to intervene in the case on David's behalf. Brett views this as a shot at redemption as he has yet to win a single case. The lawsuit soon becomes national news
David is given the identities and contact info for all of his biological children and he begins doing anonymous acts of kindness for them, including helping one son get an acting job, saving a daughter (Britt Robinson) from a drug habit, and spending time with a mentally disabled son who has been institutionalized. One of the children named Viggo (Adam Chanler-Verat) discovers David's true identity and blackmails him to spend even more time with he and his siblings. David ends up enjoying his time with his giant family and as a result, dedicates himself to being a better boyfriend to Emma. She welcomes his overtures, but notes that she thinks the man who fathered the 500 children is deranged. It is also revealed that David donated so much sperm so he could afford give his parents and family a dream vacation.
After the loan sharks beat up David's father, he allows Brett to file a lawsuit against the sperm bank for publicizing this case. At the end of the trial, Brett scores his first legal victory as the judge upholds the confidentiality clause and David's suit against the sperm bank is successful. The 500 kids plead to the news for their father to voluntarily reveal his identity, but if David does so, the sperm bank will not have to pay the judgment against them.
David goes to his father for help and his father advises him to do act as his heart tells him (he also gives David the money he needs to pay off his debts). David voluntary reveals his identity to the children (who embrace him immediately due to all the time they have spent together) and then they all run off to the hospital where Emma is going into labor. After Emma and David's child is born, David proposes and tells Emma that he fathered the 500 children. Despite some initial hesitation, Emma accepts the proposal and her new child's 500 siblings. David and Emma embrace, and their embrace is soon joined by David's brothers, father, and his 500 children.