David Wozniak (played by Vince Vaughn) is a delivery driver for his family's meat company in New York City. He's always been a bit of a slacker and never really achieved any of his goals. But one day, he gets some surprising news. He finds out that 20 years ago, he donated sperm to a fertility clinic under the pseudonym "Starbuck" and that as a result, he's now the biological father of 533 children.
At first, he's shocked and overwhelmed by the news. But as he starts to learn more about his offspring, he becomes curious about them and begins to secretly track them down one by one. He discovers that many of them are struggling with various issues, from health problems to emotional traumas, and he starts to feel responsible for them.
But when 142 of his biological children file a lawsuit to reveal his identity, David's secret life is threatened. He's faced with the decision of whether or not to come forward and reveal himself as their father. Along the way, he must also deal with his own personal issues - his girlfriend (played by Cobie Smulders) is pregnant and he's unsure if he's ready to be a father, and his family's meat business is falling apart.
Ultimately, David decides to reveal his identity to his children, and they all meet him for the first time. The experience is overwhelming for everyone involved, but David comes to realize that he's not just their biological father - he's also a friend and mentor to many of them. In the end, he gains a new sense of purpose and meaning in his life, and his relationships with his girlfriend, family, and offspring are all stronger for it.