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)