In a small shop in the city, Maia practises locksmithing in a unique way: with the help of an ancestral power and a magical door, she can repair keys that hold extraordinary secrets. Accustomed to solving every mystery without difficulty, her skills are challenged when a mysterious old woman gives her a broken key that resists every known technique. Just before giving up, Maia discovers that the object does not respond to conventional methods and that the only way to repair it is to cross into the dimension hidden behind its own door.