Complete version differences are unknown at this time. For now, please separate each console's walkthrough. Details can be combined later.
Nintendo DS[edit]
The player is racing against Mick in a cutscene. A truck appears, and Mick is involved in a crash, and he dies. Sara, as well as her wingman, confront you and tell you to figure out what caused the accident. You make your way through 6 areas, winning races. For the first 5 areas, there are 12 races, and for the final area there are 6 races. After beating a certain number of races, you get a message telling you that you can challenge the "secondary boss" of the area. Defeat the secondary boss and you will proceed to the boss himself. Defeat the boss and you will move on to the next area. Keep repeating this until you defeat the final boss.
You find out that the player actually hired EX, the final boss, to hire Buddy to crash into Mick's car, thus killing him. So after all the gameplay, you figure out it was you that caused Mick's death. The game is now over, but you can play any races you haven't already completed. Once all the races are completed, the game is finished.