The comedy story follows the title character, who is regarded by himself and others as the best hit man in Japan. He goes by the motto, If I have an assignment, I will kill anything. However, his killing assignments have expanded to include everything from summer vacation homework to housekeeping. The hit man never fails to kill the target. Though he does not aim to, he also never misses the laughter. (Source: ANN)

Kind-hearted Himuro-kun is unfortunate: when in deep concentration, stressed, or flustered, he involuntarily creates blizzard conditions for himself and anyone in his vicinity. During one such incident on the first day of his new job, Himuro-kun encounters the beautiful Fuyutsuki-san, who helps him break out of his nerve-induced ice. As it turns out, Fuyutsuki-san is his new coworker. At the office, the cool-headed Fuyutsuki-san offers simple and rational solutions to Himuro-kun's icy dilemmas—everything from helping him garden without freezing his plants to ensuring he does not melt during their tropical work retreat. Every time Fuyutsuki-san does something kind for him, the tempest of emotions he experiences inside embarrassingly manifests on the outside. As a result of the frequent snowstorms, Himuro-kun's feelings for Fuyutsuki-san are impossible to hide. Even though Fuyutsuki-san is unfamiliar with love, Himuro-kun remains determined to repay her kindness and warm her heart in any way he can.

Novice bounty hunter Chateau Dankworth never expected to encounter someone capable of overpowering her in combat. Even less so that the mysterious man would take an immediate interest in her—to the point of aiding her missions. However, Chateau makes it clear that she has no plans of entertaining any personal involvement with him. Chateau's company is soon tasked with eliminating Song Ryang-ha—an expert assassin and a former member of a powerful Asian organization. Coincidentally, Ryang-ha is the man that has been pursuing her. Despite his background, Chateau reluctantly agrees to meet with Ryang-ha as part of a deal: in exchange for going out with him, he will provide her with the locations of current targets, dead or alive. But when the past begins to haunt both killers, their arrangement may need to come to an end, as their entanglement puts their lives at grave risk.

Kotetsushin Jeeg takes place fifty years after the original and features a new cast of characters—primarily the new main character Kenji Kusanagi, a high school student who becomes Kotetsushin Jeeg to fight the sudden reappearance of Haniwa Genjin (Haniwa Phantom Gods, or clay robots) from the Jamada Empire ruled by Queen Himika. Other characters include Tsubaki Tamashiro (grandaughter of Miwa Uzuki); and Kyou Misumi. Other main characters from the original series also appear.