STAGE TUBE: Sneak Peek - Contestants Run a Marathon on NBC's THE BIGGEST LOSER

On tonight's episode of NBC's THE BIGGEST LOSER, the stakes are higher than ever when all the contestants return to the same spot where they had their first challenge to compete in a marathon. For the first time in the show's history, the contestant who wins the race automatically becomes a finalist, eligible to compete for the $250,000 grand prize. But there's extra incentive because the first five players who finish the race will also get to split $50,000. The only way to guarantee a spot in the finals is to either win the marathon - or be one of the contestants who stay above the red line at the weigh-in. In the end, the three finalists of season 12 will be revealed. Alison Sweeney hosts the series, and trainers Bob Harper, Anna Kournikova and Dolvett Quince also star.

THE BIGGEST LOSER kicked off season 12 with two new trainers - renowned tennis star and certified trainer Anna Kournikova and fitness expert Dolvett Quince - joining "Biggest Loser" veteran Bob Harper. Under their supervision, 15 contestants came together hoping to transform their bodies, their health and ultimately, their lives.

THE BIGGEST LOSER airs at 9:00 pm/et on the NBC network.

To get a sneak peek at tonight's episode, click below!