Season 12 kicks off with two new trainers – internationally renowned tennis star and certified trainer Anna Kournikova and top fitness expert Dolvett Quince – joining “Biggest Loser” favorite Bob Harper, and 15 contestants competing in a “Battle of the Ages” that will group contestants by age for the first time ever in the history of the show. Host Alison Sweeney tells the teams that they will make their first game-changing decision – who their trainer will be – in the order in which they finish the first challenge. Once each team chooses their trainer, “The Battle of the Ages” begins. The new contestants include Vinny Hickerson, who overcame a difficult childhood to become a successful Nashville recording artist, Antone Davis, a 447-pound father of four and former NFL player for the Philadelphia Eagles and Atlanta Falcons, and Bonnie Griffin, a 63-year-old grandmother of six who lost her home to Hurricane Katrina and lost her husband not long after.

Gym-time restrictions are put into place, allowing each team only two hours a day to work out. Elsewhere, the contestants receive health evaluations from Dr. Huizenga, who delivers particularly shocking news to one player; and the teams move a ball through a maze during a challenge.

Contestants from past seasons return to inspire the current players; after a challenge where the teams battle the elements, the winners receive a special bonus prize; contestants face elimination.

A player goes home and takes a trainer along for the trip. But the prize comes at a cost: the player's weight is the only one that counts for the team at the weigh-in. Elsewhere, the contestants at the ranch navigate a maze to fill a cylinder with soda; and a trainer lashes out at a player who hides behind humor.

The teams merge into one and learn that they have a chance to avoid an elimination if they're able to collectively lose 100 lbs. during the week. A candy-temptation setup complicates their efforts, however. Also: The players navigate a haunted maze and collect apples during a Halloween-themed challenge

The contestants swap teams and trainers after a calorie-counting challenge, and the realignment causes some tension. Later, the new teams face their first challenge together; and the trainers hold last-chance workouts away from the ranch.

The trainers pick one player to represent their teams at the weigh-in. Elsewhere, the contestants work under a time limit to prepare low-calorie dishes for Season 11 champ Olivia Ward and cookbook author Devin Alexander; and Dr. Huizenga delivers good news to a player.

The seven remaining players learn they will be competing as individuals and will be taking part in a pentathlon; Olympic medalist Apolo Ohno motivates the contestants; a trainer shares personal details with a struggling player.

The trainers prepare Thanksgiving dinner for the players, who also face a temptation challenge with a three-pound advantage prize for the winner. Elsewhere, a tough challenge requires the players to climb one mile on Jacob's Ladder treadmills.

The five remaining players receive makeovers with help from stylist Jeannie Mai and hairdresser Ken Paves. Later, they show off their new looks to family and friends before attending Cirque du Soleil's "Iris."

The eliminated players return to run a marathon, and the winner earns a spot as a finalist. The first five players to finish also receive a share of a $50,000 prize. Later, the contestants weigh in to determine which two move on to compete in the finals.

The winner of the $250,000 prize is crowned in the Season 12 finale. The eliminated players also return to weigh in for the $100,000 at-home prize.