Firstborn centers on teenager Jake Livingston (played by Collett), whose home life is thrown into disarray when his mother's ne'er-do-well boyfriend moves in and pulls her into his dissolute lifestyle.

Sixteen-year-old Jake Livingston's parents divorced two years earlier, and his mother Wendy has been dating other men, but she remains deeply in love with her ex-husband.

The atmosphere at home causes Jake's relationships with his friends and girlfriend to deteriorate and Brian to repeatedly get into fights at school.

When Jake confronts her, she denies her drug addiction, insists upon the fantasy that they are going to open a restaurant, and says she cannot tell Sam to leave.

As Wendy's mental state continues to deteriorate, Jake is forced to take on the parental roles in the household, including picking Brian up when he ends up in the principal's office.

When Sam gets a fresh supply of cocaine to go forward with his drug deal, Jake steals it and hides it in an abandoned building.