一名一流的杀手，他为了隐藏过去安渡余生，而更名为“源”，随后他来到了温泉旅馆“椿屋”觅职，旅馆总管舍吉见其形迹可疑而犹豫不决，而此时老板娘冴子却果断地接纳了他。事实证明，聘请阿源对椿屋有诸多益处，阿源也在各个性情敦厚的人们接纳下逐渐卸下心防，即使内心深处仍有阴冷不明的黑暗物质流动着，但总有一天过去或许真的会过去，阿源保持着这样的信念，而开始了温泉旅馆忙碌的工作人生，并协助解决了各种棘手事件。

"Yukemuri Sniper" ("The Sniper in the Vapor") stars Kenichi Endo as an assassin who has decided to leave the business and try to go straight.

Gen figures the best way to escape his violent past is to retire somewhere that is remote and quiet, so he takes a job as a lowly attendant at a mountain hot-springs resort called Tsubaki-ya. He strikes up a friendship with one of his colleagues, a housekeeper named Suzuyo (Shoko Ikezu).

One day, Suzuyo's ex-husband, Tagawa (Kohei Mashiba), comes to the inn and roughs her up. Gen learns that Tagawa is a gangster and that he shows up at the inn every so often to take the money that Suzuyo has earned. Gen feels sorry for Suzuyo and calls Tagawa to try and reason with him.