Sixteen-year-old Clara Morris is facing an awkward summer with her father in the tiny upstate town of Redmarch Lake. Clara's relationship with her parents--and with life in general--has been strained since she lost her twin sister, Zoe, when the girls were eight. As a child, her sister had been her whole world--they even shared a secret invented twin language. Clara has managed to rebuild herself as best she can, but she still feels a hole in her life from the absence of her twin, and she suspects she always will.

She soon finds that Redmarch Lake, where her father's family has lived for generations, is a very unusual place. The townspeople live by odd rules and superstitions. The eerily calm lake the town is named for both fascinates and repels her. The town's young people are just as odd and unfriendly as their parents. Clara manages to befriend the one boy willing to talk to an outsider, but he disappears during a party in the woods.

The next day, he is found dead in the lake under mysterious circumstances. The townspeople all treat this as a tragic accident. Clara isn't buying it, but she doesn't know what to do until she receives a mysterious note hinting at murder--a note written in the language she shared with her twin sister, Zoe.