Fitness and lifestyle expert, Jackie Warner, begins “Thintervention” with eight pampered clients who are unhappy with their weight. After a basic training workout in order to assess their fitness levels, the clients are told to remove all sugar from their homes. To ensure that her new policy is enforced, Jackie visits the homes of the cast the next day, only to find that most haven’t complied with the rules of their assignment. The following afternoon, Jackie introduces her extreme workout, an intensive outdoor obstacle course that pushes the clients to their limits. On day four of the program, trouble begins to brew when Jackie discovers that some clients still haven’t eliminated sugar from their diets. The new clients undertake an emotional group therapy session in which clinical psychologist Dr. Ramani helps them confront the causes of their overeating. Following therapy, the clients conduct their first weigh-in.

Week two begins with basic training, which does not produce such great results. Jackie surprises the clients when she pairs them up for the week to support each other through the weight-loss process. The newly formed teams then gather in Griffith Park for the extreme workout, where they’re tethered to each other for a grueling hike up the mountainside. At the end of the episode, the crew supports Bryan in group therapy when he opens up about a painful childhood event.

After three weeks of intensive training, the program has begun to take its toll on the clients. After a series of heated exchanges, Jackie challenges the group to meet for dinner to resolve their issues. Sadly the tension levels only continue to rise and Jackie has the crew work their frustrations out on the stairs of the Rose Bowl stadium. The exhausted clients meet for an emotional group therapy session to truly confront their weight issues. Later that day, despite their hard work, many are left disappointed standing on the scale at the weigh-in.

Jackie adopts a new method in week four's basic training, sending the team on a three-mile run through Magnolia Park. Later in the week, Jackie takes the clients through an epic firefighter workout, complete with hunky firefighters. With tension building amongst the group, a few of the clients lash out at Jackie for her strict training methods. At the weigh-in, it becomes clear that the group is divided in two between those who are consistently losing weight and those who aren't.

Five weeks into the program, several of the clients have yet to make any real dents in their weight-loss goals. To get to the bottom of this trend, Jackie assesses the intensity of everyone at basic training and drops by their houses the next day to follow-up. Later in the week, the clients get to practice mixed martial arts with acclaimed instructor Eddie Millis. Jackie discovers how several members of the group are cheating after bringing a lie detector to group therapy.

Finally, the clients have begun to see differences in both their weight and confidence levels, so Jackie presents a new theme of the week: getting in touch with their "inner sexy." The extreme workout brings this theme to fruition when Robin Antin of the Pussycat Dolls leads a sweat-filled dance where everyone has a chance to strut their stuff.

Jackie has one last week with the clients before she sends them off to work the program on their own for six weeks. In order to jumpstart their transition, Jackie takes the clients to a sports and fitness resort so she can work them out around the clock.

The clients have been away from Jackie for six weeks, struggling to use everything that she taught them to realize their ultimate weight loss goals. The clients' friends and family are brought together for a final reveal and celebration.