Fai Yan and his younger sworn sister Ko Keung become Lo Yan County Yamen constables. Newly appointed Head Captain Wong Se tells Fai Yan to go to anarchic Yat Tseung Kung City to look for some spy who has lost contact. Boon Lung Sect boss Boon Dau’s minion Moyung Gong-tsz accidentally takes Fai Yan to Yat Tseung Kung City. The city is seemingly a prosperous place, but the four major sects are covertly fighting against each other. The late sect leader promised whoever could draw the Infernal Sword, Tsui Yi Sect Big Sister Fu Ling would marry him. Fai Yan and Gong-tsz confusingly draw the Infernal Sword together, so they both become the top two prospective sons-in-law! The four sects engage in all kinds of shenanigans, and the mastermind has tricks up their sleeve. The identity of the spy is just mysterious. Fai Yan wants to be the prospective son-in-law. And the Yamen spy has to be tracked down. No one knows whether it’s a blessing or a curse.
費仁與義妹高強為工錢當上魯仁縣衙門捕快，新任捕長黃佘派遣費仁深入三不管地帶壹將宮城，尋覓失聯細作。般隆門話事人般鬥手下慕容恭梓誤打誤撞，將費仁帶入壹將宮城。城內表面一片繁華，四大門派卻暗裡爭鬥。已故門主曾承諾誰能拔出寶劍，徐意門大家姐符玲就嫁誰；費仁與恭梓糊里糊塗一同把寶劍拔出，成為兩大候選姑爺！四派互相搞局，幕後主腦陰謀不絕，細作身分謎團重重，費仁要做候選姑爺，衙門細作亦要繼續尋找，禍福難料……
