"Wadanohara and the Great Blue Sea" is a manga series that revolves around Wadanohara, a young sea witch who returns to her underwater home after a long journey. Accompanied by her familiars—Samekichi, a shark; Memoca, a seagull; and Dolphi, a dolphin—Wadanohara seeks to restore peace and balance to the ocean kingdom. The kingdom, however, is under threat from a mysterious force known as the Sea of Death, which spreads corruption and chaos. As Wadanohara investigates the source of this darkness, she uncovers a web of secrets involving her companions, the royal family, and her own past.

The narrative explores themes of loyalty, sacrifice, and the consequences of one's actions. Wadanohara's journey is fraught with emotional and moral challenges as she confronts the Sea of Death and its enigmatic leader, Ver Million. The story delves into the relationships between the characters, particularly the bond between Wadanohara and Samekichi, whose connection is tested by revelations about his true nature and his ties to the Sea of Death. Other key characters include the ocean's princess, Sal, and her brother, Mikotsuhime, whose fates are intertwined with the kingdom's survival.

The plot is driven by a series of escalating conflicts, culminating in a climactic battle that forces Wadanohara to make difficult choices to protect her home and loved ones. The manga balances moments of lightheartedness with darker, more tragic elements, creating a narrative that is both whimsical and poignant. Through its exploration of the characters' struggles and the intricate world-building, "Wadanohara and the Great Blue Sea" presents a compelling tale of resilience and the enduring power of hope in the face of adversity.