Brennan Huff (Will Ferrell) is a thirty-nine-year-old who has never left home and lives with his divorced mother, Nancy Huff (Mary Steenburgen). Dale Doback (John C. Reilly) is a forty-year-old who also has never left home and lives with his widower father, Dr. Robert Doback (Richard Jenkins). Both of them are unemployed and have been living off the goodness of their parents their whole lives. Nancy meets Robert; they fall in love, marry, and move into Robert's home. The two sons are forced to live together in the same room as step brothers. Brennan and Dale initially hate each other, but find out that they are not too different. They are like 12-year-olds trapped in adult bodies who haven't grown up and live in their own little worlds. They become best friends, and then their parents decide to kick them out, because they are getting a divorce. They set up job interviews, but to no avail. Their lives are about to change, and they must grow up.