Ranmaru Samejima and Kei Enjoji are Tokyo college students who have been friends since junior high school. Enjoji is the illegitimate son of an Osaka yakuza obaybun (don), and Samejima (or Ran-chan, as Enjoji calls him) is the "white devil of kendo," a brilliant fencer who has devoted his entire life to the dream of becoming kendo champion of all Japan.
In their senior year of high school a rival yakuza clan puts a hit out on Enjoji. Ranmaru is critically injured in the attempt on Enjoji's life. Though doctor's believe he will never walk again, Ran's indomitable bushido spirit (and Enjoji's love and support) enable him to make a miraculous recovery--although due to nerve damage he can no longer do kendo.
That's the back story. The video picks up with Ranmaru and Enjoji enrolled in their 2nd year of college, and living together. Enjoji works part time as a host (male escort) in a host club, which causes some strain on their relationship because Ranmaru is extremely conservative, and in fact, for many years struggled against the erotic nature of his friendship with Enjoji. Ranmaru works after class helping out a professor.
One evening the professor insists that Ran accompany him to a bar. At the bar he slips a drug in Ran's ice tea and begins to take advantage. Ranmaru's 'chastity' (which is quite an issue for a number of characters in KIZUNA) is preserved by the intervention of Kai Sagano, who happens to be Kei Enjoji's half-brother. Kai, another kendo fencer, has had a secret passion for Ran-sempai since high school. He inserts himself into the lives of Ran and Enjoji, much to the chagrin of Enjoji. Kai makes no pretense of the fact that he intends to win Ranmaru for his own: painful news for Mansori Araki (Masa) the so-honbucho of Shouyuukai (#1 yakuza lieutenant of Sagano elder) who with stoic devotion has practically raised Kai since infancy.
That's the story, although you may have trouble following it in this EXTREMELY CONFUSING video, which is based loosely (very loosely) on the first two volumes of the KIZUNA manga by Kazuma Kodaka. The video is very short (less than 30 minutes), lacks the subtle nuances of the original, and takes many inexplicable liberties with the plot. Still, it is something of a classic, and remains many western fans' first introduction to yaoi (the manga genre of boy love stories). Some of the art work is rather nice, and the video can be regarded as amusing supplemental material. However, if your only exposure to KIZUNA is the anime, you are missing out on an original and vastly entertaining love story.