Twin sisters, Shane (Mary-Kate Olsen) and Elizabeth Dalton (Ashley Olsen), live separately from one another on opposite ends of the country after their parents divorced years earlier. Shane, who lives with their mom in Los Angeles, is a chilled out, tree-hugging vegetarian who is very into nature and spiritual beliefs, while her estranged sister Lizzie, who lives with their dad in Washington D.C, is a level-headed grade A student with a driven personality. However, unknown to each other, they both enter to be a contestant on a TV quiz show filmed in Mexico called The Challenge, which tests contestants strength and survival skills in various tasks as they work as part of a team in order to win college scholarships.
Head intern Marcus (Brian Skala) informs the show's producer, Max (Joe Michael Burke), of their distant relationship and, knowing they dislike each other, he puts both girls on the same team, Team Mayan, as part of a deceitful scheme to boost the show's ratings. Backstage, Shane meets and becomes close with Adam (Lukas Behnken), a laid-back jock who is put on the opposite team, Team Aztec. Team Aztec also includes Kelly (Sarah Bastian), an over-competitive swimmer, JJ ((Diana Carreno)), a ditzy "triple threat" with dreams of stardom, and Charles (Ty Hodges), a calm and intuitive city boy who plans to study psychotherapy. Along with Shane and Lizzie, Team Mayan also includes Anthony (Theo Rossi), an Italian-American food enthusiast, and Justin (Zakk Moore), a surfer and skater from sunny Florida. At their first meeting, Max informs the group of the rules, which includes no romantic entanglements between teammates of the same or opposite team, as it can result in disqualification from a task.
During the first three tasks, Shane and Lizzie's bickering prevents them from working together, and Team Aztec win the points. However eventually, when Anthony and Justin make them realise that they'll need to pull together, they start putting aside their differences for the sake of the team. Over the first few days, Shane and Adam become more and more close, and Kelly suspects something is going on. At the same time, Lizzie starts hanging out behind the scenes with Marcus, although he is still being asked to spy on the girls by Max. Marcus finds out that Shane hates heights and that Lizzie is afraid of snakes, information which Max plans to use later on. Team Mayan win the fourth and fifth takes, making the score 3-2 to Team Aztec, and Shane and Lizzie both start to respect what the other knows.
One night, the girls sneak out to meet Marcus and Adam, but they're followed by Max who fails to get video footage. However, Kelly manages to take a picture of Adam and Shane kissing and shows it to Max. He disqualifies both of them from the next round, which Team Mayan eventually win, making the scoring tied. To celebrate, they are given a boat party as a prize, which Team Aztec have to cater for in costume. At the party, Marcus tells Lizzie that he's been behind Max's troublemaking and she ends whatever they had going on. Later on, the girls resolve their past problems for good after realising they're not so different after all.
The final task will crown the winners. It is an assault course featuring various challenges that includes walking harnessed on a bridge high over a canyon (Shane's fear) and then walking on narrow planks over a pit of snakes (Lizzie's fear). However, the girls help each other out and Team Mayan reach the totum before the others, making them the winners. All teammates receive college scholarships. With the experience coming to a close, Kelly apologies for the mistakes, Lizzie makes up with Marcus, and then the girls get their own back on Max by tipping slime on him while taking a picture.
The Challenge was Mary-Kate and Ashley's last direct to video production, so at the end in a short scene, past boyfriend's from their previous films show up and begin arguing over who they loved more, leading the girls to exit saying, "Boys may come and go, but we'll always have each other, and that's not just in the movies."
Shane Dalton (Mary-Kate Olsen): A laid-back vegetarian who lives with her mother in California. Very peaceful and Zen-empowered, Shane uses yoga and pilates to find peace with the world. She is afraid of heights. She wants to be an earth scientist.
Elizabeth "Lizzie" Dalton (Ashley Olsen): A modern city girl living with her father in Washington DC. The more uptight twin. She is very detailed-oriented, which proves to be a bit of an advantage during later tasks in the game. She has a fear of snakes.
Adam (Lukas Behnken): An easy-going average boy who plans to go to college to study journalism. He grows close to Shane early on the film. His parents split as well.
Kelly (Sarah Bastian credited as Sera Bastain): An extremely athletic, over-competitive girl from Knoxville, Tennessee. She is seen to have special talents in swimming, but her overly competitive nature makes her out to be a bit anti-social and a bit antagonistic of not only the other team but her own team.
Anthony (Theo Rossi): An Italian-American boy who plans to use his winnings to go to culinary school. His goal is to be like Emeril, and he is very fond of eating as well as cooking. He may be second-generation Italian immigrant, as during one scene, he thinks he sees his mother, who has a heavy Italian accent. He becomes best friends with Justin.
Justin (Zakk Moore): A typical 'surfer dude' from Orlando. Though he seems to be nothing but a fun-loving skater-boy from the start, he is shown to have a very high IQ as he plans to study astrophysics and can easily answer questions during one task pertaining pharmaceutical chemistry.
JJ (Diana Carreno): A self-proclaimed "triple-threat" with dreams of stardom. Slightly ditzy and in her own buble. It's hinted she may not be very intelligent as she initially thought she was auditioning for American Idol. In the end, she receives an invitation to be on the show.
Charles (Ty Hodges credited as Eric Ty Hodges): A very calm, intuitive boy who plans to study psychotherapy. He is the one who tries to keep his team together mentally. He also is the one who tries several times to bring Kelly back down to Earth when she lets her competitiveness get the best of her.Max (Joe Michael Burke): The arrogant, fame-hungry host of the show who will do anything to get ratings.
Sasha (Jeannette Weegar credited as Jeanette Weegar): Max's redheaded assistant, only in a few scenes.
Big Joe (Pablo Recasens): Max's right hand man, his camera man.
Marcus (Brian Skala): The head (and only) intern. He's from L.A and wants to direct films in his future.