After Callie Maggotbone refuses to go through her arranged marriage with Twayne Boneraper, her parents get back together and have a baby named Lillith that ages one year every day and takes Callie's place to marry Twayne. But when Lillith hurts her mother, she moves in with Callie and Mark. After a few days living with them Lillith is at the sex altar, where she's supposed to consummate her marriage to Twayne in front of millions of spectators. However, shortly before that she tells Mark Lilly that she plans to kill Callie and Twayne and marry Mark instead, who has been treating her like a daughter. As soon as Callie knows of that, she removes her own soul to transform into her demon form and kills Lillith at the altar.
Half-demon Callie Maggotbone's demon father Aldermach is pressuring her to break up with her boyfriend Mark Lilly and marry Twayne Boneraper in order to fulfill an ancient magical blood pact; Aldermach gets paid by Twayne's family to deliver him a bride, and if he does not, he will get murdered. Callie refuses to marry Twayne, and to prove that her relationship with Mark is serious, she invites him to move in with her, but does not tell him about her arranged marriage to Twayne.
Meanwhile, Mark is having troubles at the Department of Integration; He receives rather negative feedback on his teacher evaluation forms from his students. Since the department cannot afford having individual sessions with students, he decides to hold them at his and his zombie roommate Randall Skeffington's apartment. At his first session, Mark is kidnapped by Aldermach, who bribes him to break up with Callie. Mark declines, and Callie's human mother Rosie shows up and argues with Aldermach, which gives Mark the idea to invite them both to therapy sessions. Following the session, Mark leaves the room for a short while, and when he comes back, Aldermach and Rosie are having sex on the pool table.
The next day, Rosie and Aldermach announce that they have given birth to Callie's sister, Lillith. Thanks to the hormone injections Rosie has had, it only took a day for Lillith to be born. Lillith will also age one year every day, and take Callie's place as Twayne's bride. After an accident caused by Lillith, Rosie is hospitalized, and Mark and Callie take on the task to raise Lillith.
In just a few weeks, Lillith is eighteen years old and has started dating Randall. Mark takes in Twayne for a therapy session and finds out about his arranged marriage to Lillith, and that the marriage will be a televised sex ritual. Callie realizes that she is forcing Lillith to marry Twayne, just like her father forced her, and tells Lillith that she does not want her to marry Twayne. Lillith, however, does the opposite and says she will marry him.
Just before the ritual, Lillith reveals to Mark that she plots to kill Callie and Twayne and instead join Mark at the sex altar. Mark informs Callie, who removes her soul to transform into her demonic form. Following a battle between the two, Lillith is defeated and falls into to a deep circle of Hell. Later, Callie kicks Mark out of her apartment, as she is no longer required to marry Twayne.