If you don't already know Reni from GTA Libery City Stories then you get a comically explicit introduction here. Your mission, and you have no choice but to accept it, is to drive around a stunt course for Reni as (s)he films you for a commercial from a helicopter above you.
Take care not to crash too much and destroy the car too quickly, you'll be doing enough crashing as required for the course. Slow down for the first couple of turns, then you'll get more instructions to hit the stunt cars for bonuses. The stunt cars are marked on the map as red dots, and one will be coming towards you very soon, so swerve into it for bonus time. Head around the course following the markers and hitting any stunt cars that come towards you. Soon you'll also have some ramps to jump too.
Not long after you pass Washington Beach police station you'll get a message about hitting the last checkpoint at the right speed. This checkpoint will be a sharp right turn into an alleyway a little to the south. As long as you're accelerating as you enter the alleyway you should get enough speed up. The final shot sees you jump the ramp and smash through the window of an apartment in the hotel opposite.