Eighteen contestants are dropped in the middle of the desert as Season 4 begins. This year, in addition to the usual red and blue teams (trained by Kim Lyons and Bob Harper, respectively), a secret black team is formed, trained by Jillian Michaels.

The immunity challenge has the red and blue teams racing up the stairs of a 10-story building. Then, the secret black team is revealed in the weigh-in room, creating a three-way competition.

The teams are put to the test for the first time with a tempting food competition. Each contestant will need to decide whether it is worth it to participate when they are put in a room full of unhealthy food. They have four minutes to consume or not consume the most calories and the winner will receive a three-pound pass that will be used to their advantage at this week’s weigh-in. In the physical challenge, contestants will participate in three different 100-meter heats against the yellow team formed by three kids. Host Alison Sweeney informs them that the winning team will receive a phone call home, making the stakes this week even higher. After the weigh-in one of the teams will have to send someone home.

During the food temptation, the trainers are tested alongside the contestants. If Bob, Kim or Jillian give in to a temptation, their team receives a 15-second advantage in the physical challenge. For the physical challenge, the contestants must race up and down a hill to their team flag.

The teams are surprised with an exotic getaway, but they soon learn their vacation is more work than play. Their physical challenge is unlike anything they have had to face before, and in the end one person is sent home.

The remaining teams compete in a special challenge testing the contestants' perseverance, with the winning team getting a surprise from home. After the weigh-in, one contestant is sent home.

The contestants play a version of poker, and the winner gets a prize that will change the game. After a dramatic weigh-in where one contestant shocks the others with their weight, one contestant is eliminated.

In the aftermath of last week, 's surprising game play by one contestant, the other players - and the trainers -- try to move on and bring the focus back to weight loss rather than gameplay. This week's challenge tests the endurance of all the teams, and host Alison Sweeney tells them the prize will give the winning team a great deal of control over this week's competition. Then it, 's the most emotional elimination yet of the season, followed by a very emotional homecoming when the eliminated player returns home to family and friends.

The red, blue and black teams all go , green, for the week, participating in recycling and learning how to implement new environmentally friendly habits in their lives. They also learn to exercise without the aid of electricity, taking their workouts to a whole new level. This week, 's challenge is all about recycling, and the winners get something really environmentally friendly , Ford Escape hybrids!

Players begin competing as individuals, and are faced with a temptation where one players is overcome by the donuts. Later, they compete in a triathlon challenge, and the winner gets to go home for 24 hours.

The contestants head to the set of Days of Our Lives soap opera, where friends and family wait to surprise them.

The viewers will choose this week's challenge for the seven remaining contestants.

The remaining six contestants are put to work in a pizza place. Later, they participate in a contest involving punching.

The five remaining contestants look back at their hard work up to date before one more is released from the show.

In the live finale, The Biggest Loser contestants from this season return for a chance to weigh-in one more time. Later, one of the final four is named the season's Biggest Loser.