Choi Woori has spent the last three years living alone, and the only remedy to her loneliness is the company of her five-year-old poodle mix, Maru. But one morning, something miraculous happens: standing on her bed is a five-year-old girl calling her "big sister"—Maru has turned into a human child! Woori barely has time to adjust to the whole situation, as Maru has kept some of her puppy-like habits. She still tries to bark, she grabs stuff with her teeth, and their daily walks are slightly more challenging without a leash. With her boundless energy and irresistible charm, Maru often attracts the attention of their somewhat eccentric neighbors, promising that loneliness in Woori's life is now a thing of the past. [Written by MAL Rewrite]