One night, Alex takes Cheryl to a "party" that promises something exciting to happen to the participants (presumably overuse of drugs). Cheryl is worried by leaving Grace alone in the car; therefore, Alex leaves to check on her. A few minutes later, a police detachment arrests every one of the criminals.
One night, Alex takes Cheryl to a party that promises exciting events for its participants (presumably involving drug use).  Cheryl is worried about leaving Grace alone in the car, so Alex leaves to check on her. A few minutes later, law enforcement officers arrest everyone involved in the criminal activity.