Spoiler:
How the Game Ends
Well, there is a very thin plotline in the Underground Mode part of game
(which just fills in the gaps between the action), and it has one
noticable hole; after you race Samantha, she's supposed to be gone because you've
betrayed her trust, but you can still hear her rave about it when your car makes it
into another magazine cover. Beyond that small plot twist, it's pretty monotone all the way.
Here is the spoiler, how the game ends. Amazingly, the plot has another twist;
The Underground Mode claims to have 111 challenges that you have to complete, the 111st being a race against
Eddie, the number one driver of the scene. But then, there's race 112! After winning against Eddie, an anonymous shows up in
a Nissan 350Z and he wants to race you! After winning this one, the cutscene shows how the
anonymous -- whose name Melissa we could see anyway during the race -- is actually a
hot chick, and you see her stepping out of a bright beam of Nissan lights towards you.
She wants you! Meanwhile, the female voice of Samantha is raving "I can't believe it,
you won, you won!", and noticing Melissa, "is that your fantasy?".
And so the game ends and brings you back to reality; you're no hot street racer, you
just fantasize being one. And no, you're not scoring with that chick you just beat in a 4-lap, 200 MPH race.
I'm not sure if the game developers meant it to be seen that way, but I find it sarcastically honest.