From MangaHelpers:

In London of 1882, an injured woman safeguarding a powerful ring is fleeing a dangerous organization. The ring is pickpocketed by a local thief gang, and she passes out from her injuries before being able to recover it. When she awakes, she finds that the young thief gang has taken care of her, and a boy named Sari, who has no memories of his life before the gang took him in five years ago, has the ring.

When the organization tracks them down, he wears the ring and unleashes its power. However, he can't take it back off. Now the woman wants him to come back with her to her organization. Will he uncover the secrets of his past? [tethysdust]