High-school student Matt Leland lives with his twin brother and sister and his father in a house by the lake. When teenager Casey Roberts moves into the house on the other side of the lake, Matt snoops into her room with his telescope. They meet and soon they fall in love. One day, Matt is taking a test at school and Casey activates the fire alarm to stop the test. She is suspended and her father decides to check her into a psychiatric institution, but Matt breaks her out of it. They travel in Matt's car and have lots of fun until the day Matt discovers that Casey has bipolar disorder with periods of depression and periods of elevated mood. When she tries to commit suicide, Matt realizes that she needs specialized help.
