Mini Drogues (Nikki Reed) is a clever and adventurous high school senior who is bored with her life. Mini prizes her "unique experiences" (she calls them "firsts"). For excitement, and to add to her list of firsts, Mini decides to try being a call girl. Her first client, however, has a guilty conscience and can't carry through with the act, which disappoints Mini. Her second client is decidedly more exciting: her stepfather Martin (Alec Baldwin). Martin is initially shocked when he learns of her identity (he initially blindfolded himself during intercourse as per Mini's request), but soon a torrid love affair blossoms between the two.
In order to be together, Mini and Martin concoct a plan to have Mini's mother Diane (Carrie-Anne Moss) declared insane. When their plan fails, Mini convinces Martin to murder Diane, despite his initial resistance to the idea. They attempt to make it appear that Diane committed suicide, but they soon attract the attention of a detective (Luke Wilson) who believes that Mini and Martin killed her. A nosy neighbor, Mike (Jeff Goldblum), is sexually obsessed with Mini, and when Martin learns that Mini had gone to Mike's house and had received sexual pictures from him, he and Mike get into a fight. Mini arrives to find Martin standing over the neighbor, ready to beat him into unconsciousness, and when the police arrive they arrest Martin.
Mini visits Martin in jail and admits that the sexual pictures sent were actually from her in order to get Martin to think that the neighbor sent them. She also reveals that she assumed the police would eventually believe he killed Diane (since he was the more likely perpetrator). Mini, therefore, ends up getting away with murder, and inherits her mother's fortune. The film ends with Mini giving a valedictorian speech, even though she is a C student; the school gave her straight A's out of sympathy for her mother's death. She offers advice to the graduating class about how to live a good life, that perversely alludes to her crimes without making her look too suspicious. The detective is present at the speech, clearly still suspicious of Mini, but knowing that he will probably never be able to prove that she was guilty of murdering her mother.

Question: Why was Martin arrested?
Answer: He was arrested for trying to beat Mike unconscious.
Alternative answer: Killing Diane