With the big company intramural football game around the corner, everyone starts getting nervous about the lineup -- Mark is a wonderful team player, but a terrible actual player. When Randall demonstrates the power of his recently replaced arm, he is given a job at the D.O.I. as the team's ringer.
An ant-sized arsonist named Jerry flees Mark's custody, escaping up Twayne's nostril. A miniaturized Leonard, Grimes and Mark must track Jerry through Twayne's body to bring him to justice.
With Mark working all the time, Randall feels neglected, so he decides that they should spend more time together to decide if they should stay roommates. And Grimes's Mummy (Christine Baranski) returns from the dead, only to find fault with everything Grimes is doing.
Callie reveals to Mark that she's been sleeping with other guys. So when Mark's college girlfriend (Jenny Slate) shows up, he decides to put his newly "open relationship" with Callie to the test.