A young Taiwanese assassin codenamed "Glass Heart" committed suicide by jumping off a building, and her heart was pierced by a metal fence. Miraculously, her life was saved by heart transplantation. During her recovery she began to experience strange dreams, which led her to Japan looking for the donor of her heart, who happens to be Kaori Makimura, former partner of City Hunter.
This manga is a follow-up and alternate history to the series "City Hunter", diverging from the previous series with a "what-if" and going in a very different direction. That said, it isn't necessary to read "City Hunter" first; I didn't, and had no problems understanding what was going on. It's a seinen manga, so it deals with adult subjects, including a lot of sexual humor, but there isn't much that's explicit; the most fanservice-y it gets is in the full-page title page illustrations, as is normal.
I liked that this series has an emotional breadth. It's sometimes a comedy, sometimes full of action,
and will then turn round and wrench your heart out with its emotional side. Ex-assassin Shanin is as mature as her backstory would suggest, but then something happens to remind one of just how young she is (sixteen at the beginning of the series). Ryo Saeba, the protagonist of City Hunter, plays a major role here too, as he finds himself feeling responsible and even parental toward Shanin, and has trouble reconciling that with the dirty-minded immature teenager persona he's become so used to playing in public. Everyone has a lot of character development to do, and it happens, believably.
Most story arcs involve cases that Saeba and Shanin take on as private investigators, and often they don't appear at first to be actually advancing the main plot of the central characters, but generally in some way or other they do move things along. The pace is slow with the central plotline, yes, but realistically so - one gets a feeling that, like in the real world, things can take sometimes years to work out.
Recommended as long as you can cope with probably having to wait a number of years for the story's conclusion.