It serves as the main storyline of Need for Speed: Underground. Upon entering it, players receive a welcome message before starting. From the Underground Menu, players have access to the race map, customisation, rankings, magazines and their overall progress in the game. Throughout this mode, the player will need to defeat several characters including Jose, Dirt, Klutch, and The Eastsiders. At the end, the player will race against the leader of The Eastsiders, Eddie. After defeating him, the player then races against his girlfriend Melissa in one final race.
Underground Mode features a total of 113 races. It features all race types including circuit, sprint, lap knockout, drag and drift. Tutorials for drag and drift can found in the Quick Race menu. Underground Mode also includes tournaments, which sometimes require players to meet certain requirements before competing. Bank rewards become higher as players progress through the career. With each race, players have the option of playing them either on easy, medium or hard difficulty, as well as in Quick Race. Also included in the Underground are time trials given by Samantha or TJ, allowing players to win unique visual or performance upgrades.
Once players beat a few races at the beginning, visual and performance upgrades become available for purchase. Visual upgrades include body parts, accessories, paints, vinyls and decals. Performance upgrades include engine, ECU, turbo, transmission, suspension, weight reduction, tyres and nitrous oxide. Throughout Underground Mode, players will need to acquire visuals to increase their visual star rating, and also purchase performance upgrades in order to stand a chance against tougher opposition. This is required to enter certain tournaments as well. The higher a car's star rating is, the more style points players will earn.
As players progress through Underground Mode, they will also gain magazine covers. There are a total of 27 magazine covers to obtain. They are won by reaching a certain ranking, obtaining reputation stars or purchasing certain performance or visual upgrades. Four of them require players to win certain race types (sprint, circuit, drift and drag) on hard difficulty and beat a record.
Throughout Underground Mode, players will need to reach the top of the rankings for each race type in order to be the best racer. Players can view these statistics from the Underground menu to see how they stack up against other racers.
From the menu, players can view statistics for each race type, as well as overall statistics for the whole game, including wins, losses, win percentage, style points, bank and mileage.
After completing Underground mode, players will receive a congratulation message for completing it. In order to start another Underground Mode, players will have to create a new profile. However, all visual and performance upgrades become available in Quick Race upon completion.
Underground Mode's introduction is a circuit race using the "Vortex" Acura Integra Type R in Olympic Square. After its completion, a cutscene with Samantha is followed by the player's choice of their starter car.