Rachel Marron is one of the most famous singers around but is unaware that a stalker has been sending her deranged and disturbing death threats. She continues to perform while her manager searches for a bodyguard that can protect her ("Queen of the Night). Frank Farmer is a former Secret Service Agent hired to serve as Rachel's bodyguard. Frank manages to get into Rachel's mansion without needing any clearance and explains that security needs to be heightened, but Rachel is oblivious to her stalkers threatening notes and believes that she is perfectly safe. Frank watches as Rachel rehearses one of her songs with her young ten-year-old son Fletcher ("How Will I Know). Afterward, Frank is introduced to Rachel's existing bodyguard, Tony, who believes that he is good enough and that Rachel doesn't need Frank to protect her.
Rachel works on a new song with her sister, Nicki ("Greatest Love of All"). Meanwhile, Frank equips Rachel's mansion with security, and Fletcher imprints on him, looking up to Frank as a fatherly figure. Rachel sees them interacting and goes up to Frank and tells him that she doesn't want him to interact with her or her family. Frank leaves for the night to see Nicki perform at a small club. After performing, Nicki sits down with Frank and shares a drink with him. She tells him about her childhood band, and that after Rachel joined it became clear that there could only be one singer in the family. Frank gives a chilly Nicki his coat, and Nicki flirts with him, hoping to begin a romantic relationship with him ("Saving All My Love for You).
Sy, Rachel's music producer, pressures an angry Rachel to perform at a nightclub called the Mayan. Rachel ignores him and angrily vents about Frank, who won't let her go to Sunday Brunch. Frank tells Rachel that she can't do anything that she's normally done, like weekly Sunday Brunch. Rachel asks Frank if she can perform at the Mayan, but Frank tells her that it's too dangerous. In an effort to anger Frank, Rachel tells Sy to book the show at the Mayan.
The night of the show at the Mayan approaches and Rachel's stalker prepares to kill her. Rachel arrives at the Mayan to find a death threat in her dressing room. Frank reveals to Rachel that this isn't the first note and that a stalker is trying to kill her. Frank tells Rachel that they should leave, but Rachel declines, explaining that she doesn't want to be a coward. Rachel gets on stage and performs, but the stalker comes on stage and almost stabs her, before being thrown off stage by Frank. Frank swoops Rachel up into his arms and runs out of the club with her, saving her from death. ("Mayan Medley"). Back at home, both Rachel and Nicki wonder about their feelings for Frank ("Run To You).
Rachel begins to be more cautious and asks Frank out on a date since she can't leave the house unless he's with her. Frank reluctantly agrees, and the two go to a small karaoke bar, where a group of college girls performs a hilarious and broken rendition of "Where Do Broken Hearts Go". Rachel tries to figure out Frank's life and says that if she gets him figured out then he has to sing. Frank agrees and Rachel guesses incorrectly, but Frank decides to sing anyway. He goes up to the stage and sings a shaky rendition of Dolly Parton's "I Will Always Love You. Rachel goes up after him and surprises the audience as they realize that she is the real Rachel Marron. Rachel proclaims her love for Frank and the two embrace and share a passionate kiss ("I Have Nothing).
Rachel wakes up in bed next to a sleeping Frank, presumably after sex. Nicki sees this and grows jealous of her sister. Soon afterwards, Rachel works on a new song in the recording studio ("All the Man That I Need).
Frank later gets a call from his colleague Ray Court that he needs to break off his relationship with Rachel, explaining that the stalker followed them to the karaoke bar and stole Rachel's scarf. Frank tells Rachel that he was reckless and can't be romantically involved with a "client". Rachel is extremely mad and storms off. Frank comforts a troubled Fletcher by giving him a clearance badge for the night of the Oscars. Frank then leaves with Rachel for a benefit concert in Miami, Florida, where Rachel performs a splendid routine ("I'm Every Woman").
Frank checks in with the temporary bodyguard who is watching Nicki and Fletcher. Everything is presumably fine, but Frank is unaware that the temporary bodyguard is actually the stalker. The stalker tells Fletcher that he needs help with Rachel's "present", and Fletcher eagerly goes to help him. Nicki sits in her room thinking about her unrequited love for Frank, but upon going inside Fletcher's room she sees that he has disappeared and in his place on the bed is one of Rachel's dresses, stolen from her dressing room ("All At Once"). Nicki calls Frank and Rachel and they return home from Miami. Fletcher is alright, but the stalker took his clearance badge for the Oscars. Realizing that her son and sister are in as much danger as she is, Rachel tells Frank that she will do whatever he says if it will keep them protected. Frank has Rachel cancel the rest of her tour and the four of them travel to Frank's deceased parent's cabin ("Run To You (Reprise)"). Frank confides in Rachel that this cabin holds lots of memories and that he misses his father and his mother, who he wishes he had spent more time with before they died. Frank watches happily as Rachel shares a tender moment with her sister and son ("Jesus Loves Me").
That night a drunken Nicki admits to Frank that she wrote to the stalker pretending to be Rachel in order to feel attention. Nikki now believes that it is her fault that all of this has happened, and Frank comforts her and promises to catch the stalker. Soon afterward the stalker breaks into the cabin and approaches Nicki. She believes that it's Frank, but screams in terror as the stalker fatally stabs her ("Nicki's Death). Frank, Rachel and Fletcher run into the room and find Nicki on the ground. The stalker runs off and Frank chases after him, trying to shoot him down. Meanwhile, Rachel tries to keep her sister alive but weeps as she dies in her arms ("Jesus Loves Me (Reprise)").
Frank, Rachel and Fletcher return home. Frank comforts a fearful Fletcher and asks Rachel if she is still going to attend the Oscars. Rachel tells Frank about her and her sister's dream of winning an Oscar and decides to do it for her sister.
At the Oscars, Rachel performs "One Moment in Time" from the Oscar nominated film Queen of the Night that she co-wrote with Nicki. As she performs the stalker prepares to shoot her. Frank notices just in time and jumps in front of Rachel. He gets shot, and Rachel holds him in her arms, uttering the famous line, "Let him go, he's my bodyguard!".
Frank returns to the mansion sometime later, still recovering from a shot to the arm. Frank shares a tender goodbye with Fletcher and Rachel, who kisses him goodbye as they presumably go on another tour with a new bodyguard ("I Will Always Love You").