It has been two weeks since Nepta and his sixteen-year-old sister, Coralie, visited the earth world. Now Coralie is ready to leave for boarding school. Wizen, the old merman, is accompanying Coralie and her mother, Meerlyn, on their journey. But as the shell carriage arrives at the school, no one knows what is in store for Coralie. After her mother and Wizen bid her farewell, Coralie quickly learns that her room overlooks a haunted kelp forest ruled by the Alptraum. Despite warnings to stay away, curiosity leads Coralie to enter the shadowy forest where she is welcomed by those who say they have been waiting for her. After she meets a prince confined to the forest by his dangerous nephew, Darkla, he gifts her with a strange magical power. As Coralie is led on a journey into the unknown where she must plot against Gwilda the white witch, she travels between reality and the forest world as her friends and family set out through turmoil to find the truth and hopefully save Coralie from herself. Checkmate shares the exciting conclusion to a trilogy as a teenager enters a dark forest and into a destiny she never expected.