Okajima Rokuro is a small-time salary man who is carrying documents for his company, when the ship he's traveling on is attacked by pirates. Kidnapped, he discovers to his dismay that his employers' main concern is to ensure the documents don't get into the wrong hands, even if it means sending the carrier to the bottom of the sea. Now, with his former life ruined and his kidnappers seeming comparatively friendly, "Rock" decides to join their merry band of mercenaries, and sets out with a new career to the shadier corners of the South China Sea.
Both Gunsmith Cats and Zero In are those "girls with guns" (and sometimes with swords or explosives) series. The girls in Gunsmith Cats are bounty hunters, while those in Zero In are police officers. Zero In features those (ridiculous gun-fu like) weird combat skills, on the other hand Gunsmith Cats aims for more realism when it comes to combat, but there is a bit of explicit content there. There are some chances that you might like one of those if you liked the other so, you might as well give it a try.