Twelve year-old Callie Rich is eavesdropping on her father, Jared, and her mother, Susan, speaking with her uncle, the Prophet and leader of their polygamist church, the Lambs of Zion. Callie’s sister, Alyssa, has been found murdered and Abraham will be holding funeral service the next day for the entire town to express their condolences, but after her parents refuse to attend, two guards arrive on their doorstep with instructions for Jared to pack up and get out. He’s become one of many men within the church to be expelled.

Desperate and broke, Jared seeks out the help of private investigator Max Barker to help him get his family back. Max, looking for a way to make good on a broken promise to Jared's son, Zeke, decides to help him.Meanwhile, Callie has been declared the Prophet's next wife. Her distraught mother will be married off to Abraham's second in command, Bishop Caleb Dobbs. Susan spends the next few days teaching Callie all she knows about being a good wife, as well as sewing her wedding dress.

Max and Jared arrive at Zeke's home. He was excommunicated from the church years ago and is unsympathetic to their plight, as Jared had abandoned him. Jared attempts a reconciliation, but Zeke is unreceptive and kicks the two men out of his home. But Jared has a plan. He remembers his laptop is still sitting in a safe room in his house. On it are videos of Abraham's wedding night activities, including the ones he'd performed with his underage brides. If Jared can get the laptop, he can bring it to the authorities and have his brother arrested, saving his family. He plans to get in via the secret escape tunnel running beneath both his house and his brother's mansion while Max drives into town to provide a distraction.

Unfortunately, Jared discovers that the laptop has been stolen. Back outside of town, he reconnects with Max who has taken a cell phone with incriminating photographs belonging to one of the Prophet's guards.Callie is struggling to celebrate her marriage to the Prophet at a reception held at the mansion. She meets three of her new sister wives: Edith, Linda, and Patty, who take to her right away.

After the party, Callie is ushered upstairs to a room at the end of the hall where she is expected to offer herself to her new husband. Instead, she urinates all over the bed incurring his anger, but sparing her from being assaulted. Abraham says that as long as she is still a child--meaning she hasn't menstruated--he will not consummate their marriage.

Over the next several weeks, she finds her place in the house among the other wives. As her relationships with Edith, Patty, and Linda grow, they begin to trust her more and bring her into their inner circle where she learns of their plan to escape from the house, get out of town, and build new lives for themselves far away from Abraham Rich.

Max and Jared take the phone to an FBI agent who tells them it's inadmissible, but if they were to provide evidence of probable cause, a warrant could be obtained to search the mansion. Zeke and his father have a long, drawn out fight about what had transpired at the church years ago and eventually reconcile. Zeke agrees to testify for the FBI while Jared plans to infiltrate the mansion to collect the evidence they need for a warrant.

The wives go about their house duties during the day and plan their escape at night. One night, Jared catches his daughter coming out of a meetup in the mansion's basement and gives her a set of small cameras to hide around the house. He'll watch over them from the outside while gathering evidence for their case.

Days later, Abraham announces he's taking a week-long trip to visit his new church compound in Texas. The wives are ecstatic until Callie begins to experience her first period. The countdown has begun. The wives speed up their escape plan before Abraham gets home, but Bishop Dobbs learns about it and kidnaps Callie. Jared sees what's happened and rushes back through the tunnel to save her, but he's also caught. Abraham returns from his trip early to confront his brother and niece, confessing how he murdered Alyssa after she tried to escape. Susan is brought in by the guards for one big family reunion, but she's already met up with Linda and the gang upstairs. With the knife concealed behind her, she stabs Abraham in the arm, distracting him long enough for her and her family to escape. Abraham flees through the tunnel only to catch up with Max and Zeke who've been waiting.

Zeke pulls a gun on him and Max talks him down, but Abraham gets the jump on him and stabs Max in the back. As he lays dying, Zeke forgives him for not being able to help him when he was younger. The FBI shows up and arrests Abraham.One year later, Abraham is in jail, while some members are still trying to keep their church alive. Callie's family, as well as Linda, Edith, and Patty are all living new lives far from the Lambs of Zion.