In 1953 New York, brave and clever little girl Mopsy accidentally meets her long‑lost twin brother Nico. To break free from the control of villain Vera, the two children take a risky chance to swap identities. Mopsy sneaks into the wealthy manor to gather evidence, while Nico stays with their birth mother Claire and experiences warm family love. With the help of messenger pigeon Goldie, the siblings outwit one danger after another, gradually uncovering secrets of the hospital fire and their origin. They protect each other in pursuit of truth and happiness.
