She just wants to start fresh so she can graduate in peace. She just wants to leave behind all the bad memories - her sleepwalking, the trauma from being bullied at school - all of it. She did everything she could to keep to herself, but the class president, Hae-on, just won't leave her alone. One day, when Seulbi visits the local lake Hae-on recommended, she comes across something she never imagined seeing in her wildest dreams: a wild, dark mermaid.