Song of the Deep follows Merryn on her journey to save her father deep under the ocean. Her journey takes her through fantastical landscapes and frightening trials. The friendly creatures she meets along the way show her the beauty and fragility of the ocean. This is contrasted by an ancient human society known to have inhabited a place called the Forbidden City. Their mistakes of the past still threaten the oceans today. Merryn must fight this ancient menace through merfolk ruins, underwater lighthouses, and the deepest depths of the undersea abyss. In order to stop the ancient threat and save her father she must grow her abilities, find her place within the ocean, and help the ecology of the sea.