1. Namae no nai Uma (A Horse with No Name) 
The story (the prequel to MARS) takes place after Rei and Tatsuya enter High School. Tatsuya, who at the time has a crush on Kira, dislikes Rei and picks fights with him. After the climatic motorcycle chase between Rei and a reckless gang of bikers, Tatsuya and Rei begin a new friendship. Other main characters of MARS like Harumi are also presented. 
The other two chapters are earlier works of the author non-pertaining to MARS: 
2. Nemuru Raion (Sleeping Lion) 
Tells of the strange relationship between Isomu, a guy who pretends to not have any fear, and Ayame, a girl with particular behaviours. 
3. 1 Karatto no Kajitsu 
Tells of Nina, who has some family problems, and Rui, her classmate who is interested in a close friend of the hers. But what if he's interested in Nina? 
(Source: Anime News Network, edited)

Kira, a shy high school student, lives only for her art. Rei, an arrogant, rebellious and violent playboy, wears his delinquency like a badge of honor. They are exact opposites in every way, but when Kira sees Rei kissing a statue of Mars, she overcomes her fears and asks him to model for her. And, to everyone's surprise, Rei agrees.
 
(Source: TOKYOPOP)

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