At the edge of the city, in the ruins of an old amphitheatre, there lives a little homelss girl called Momo. Momo has a special talent which she uses to help all her friends who come to visit her. Then one day the sinister men in grey arrive and silently take over the city. Only Momo has the power to resist them, and with the help of Professor Hora and his strange tortoise, Cassiopeia, she travels beyond the boundaries of time to uncover their dark secrets.