A corrupt, philandering Hong Kong businessman, Chan Ming-fu, finds himself the target of a ninja clan after launching a deal involving a Japanese company. His wife is angry over his unfaithfulness - and his sister-in-law, Sun Chi Mei, competes against his business ventures. Chan hires Chow Han Wei to protect him from the ninjas. Chow is a kendo teacher with ninja training, who begins a romance with Chi Mei, an accomplished fighter in her own right. Sun Chi Mei enters Chow's fencing school clad in red cloak, white blouse and tight black leather pants, and challenges him to a sword fight to see if he's good enough to teach her. Later, she has a fight in a bar with a group of rowdy men trying to grope her drunken sister. A persistent police detective investigates murders of various associates of Chan Ming-fu and asks for help from Chow Han Wei as well. A mysterious Female Ninja infiltrates Chan's business operation and succeeds in bugging his office to monitor his plans. Chow manages to fend off ninja attacks on his client, his girlfriend and himself, in an array of ninja weapons and techniques, and he eventually takes the fight to the Ninja Clan's elaborate cave headquarters on an island near Hong Kong, where he engages in a frenzied sword fight with black-clad ninja attackers, and finally takes on the elusive Iga Clan's Leader in an intense one-on-one kung fu bout. There are flashbacks to Chow's own ninja training as a youth with a Japanese teacher who was slain by the Iga ninja chief. One of the ninja killers presented as World Wrestling Champion) wear little red briefs, takes on a careful of men and their car, in an underground parking garage. He later engages in a grueling fight with Han Wei.