When Vance Landon breaks his engagement to Jill DeJong a week before Christmas, her world falls apart. She can't understand the reason why he does it, and he refuses to talk to her about it.
Jill decides to see his father, Edward, in a Richfield, to find out what has happened. Edward turns out to be a lecherous, but rich old man, and he doesn't know or care where Vance is or what he is doing now. However, Edward gives her the name and address of Lonnie Michaels, Vance's former roommate, who might know more.
Lonnie, who also doesn't know where Vance is or how to get into contact with him, is smitten with Jill. After she leaves, he hears voices, which he thinks are God's voices. The voices tell him that Jill is the one he's supposed to marry, and together with her, he will be the savior of the world. Lonnie hatches a plan to see Jill again.
Since Vance has only a few classes left to graduate, he decides to stay in Cedar but finds another apartment, since he doesn't want Jill to confront him when she returns. Jill’s parents told him that Jill is his half-sister, and made him promise not to tell her.
After visiting the grave of her mother's sister in the Richfield cemetery, Jill returns home and has a sad Christmas. Jill's twin, Jenny, talks her into going to the church's New Year's party with her. There Jill meets Lonnie again, but she leaves the party early, because of a headache.
At home, she has a conversation with her mother, who doesn't seem to understand her feelings, and thinks that Vance is not good anyway.
At the medical company he's working for, Lonnie asks for, and gets a transfer to Cedar. He calls Vance, and they decide to room together in the new place Vance found.
After classes start again, Lonnie meets Jill on campus, and talks her into going to a movie with him. They have a good time, and after the movie decide to eat at a Mexican restaurant. Lonnie tells Jill about his plans to become a preacher and to save mankind from evil. On the ride home, he reveals that she's been chose to be his helpmate, and tells her she should pray about it.
Lonnie tells Vance that he had a date with Jill. Vance decides to see his father, to try to make up with him, and Lonnie takes the opportunity to make some preparations. He goes to Steamboat Falls, an abandoned mining town, where he chooses a sturdy house and reinforces the boards over the windows and doors. He leaves the back door unshuttered, but installs a new lock. Inside, he prepares a bed and places candles and a space heater along the walls. After making sure all is ready and the place is secured, he returns to Cedar.
The next Friday, Jill steps out of class into a heavy February snow storm. She's wearing a tight skirt and high heels. When Lonnie shows up and offers to drive her to her dorm room, she accepts.
Instead of taking her home, he takes her to the house in Steamboat Falls, where he tells her they are married now and will have their wedding night. Jill can't convince him that she doesn't want to marry him, but she's able to talk him into having a proper marriage ceremony. Lonnie decides to marry them himself, since he is a preacher. He'll return the next morning with a witness, to make it all legal.
Jill finds a hole in the roof in the attic. She rips the bed sheet into strips and tries to climb out, but loses the sheet and one of her shoes. With an effort, she can pull herself back into the attic.
When Lonnie returns to his apartment that night, Vance is wondering what is going on. He follows Lonnie early the next morning and sees him buying a wedding cake, put it into his station wagon and leave. Vance follows him to the house in Steamboat Falls, where Lonnie catches him and ties him to a chair with an extension cord.
Lonnie finds Jill in the attic and forces her into the living room, where Vance sits, all tied up. Lonnie performs a wedding and when Jill refuses to say "yes" at the end, he declares them married anyway. Jill escapes to the back door, but Lonnie catches her, hits her, and carries her to the bedroom. Jill convinces Lonnie that she needs to eat something before they go on, or she'll be sick.
Vance manages to get free and rushes to the bedroom. Lonnie and Vance fight. Lonnie wins and Vance lies stunned on the floor when Jill attacks him with a chair and lays his cheek open. Lonnie pulls out a knife to teach Jill a lesson. Vance saves Jill from Lonnie's knife, and they fight again. Jill gets a hold of the knife. Together, they overcome Lonnie, but while Jill gets the extension cord to tie Lonnie up, their fortunes reverse again, and she finds Lonnie with the knife, bent over the prone Vance.
Again, they manage to overcome Lonnie, but before they're able to tie him up, they realize the house is on fire. Lonnie breaks free and disappears into the smoke and fire.
Vance and Jill crawl along the floor toward the back door. They see Lonnie kneeling in front of the table with the wedding cake, his hair on fire. Vance crawls toward him and returns with the keys. Finally at the back door, Jill unlocks it, but Vance yells, "no." Too late. Jill opens the door and Vance tells her to run. The house explodes.
Jill returns to consciousness outside the burning house and finds Vance severely injured. She helps him into Lonnie's station wagon, but they don't have the keys. Jill borrows Vance's shoes and slogs down the road to get Vance's pick-up. She manages to get him to the hospital.
Vance wakes in the hospital, realizing he has his father as a roommate. Edward is in the last stages of cancer and asks Vance to forgive him. He assures his son that what he had discovered about Jill was correct. After he dies, Vance receives a phone call from Jill's father, who says he has something important to tell him.
It's the twin's birthday. Their parents tell them the story of their mother's sister, who died in childbirth. One of the twins is her child and the child of Vance's father, the other one is The DeJong’s natural child. Mrs. DeJong swore to her sister she wouldn't reveal that secret until her child turned 21. The last Christmas, when Vance came to ask for Jill's hand, Mr. and Mrs. DeJong told him that he's Jill's brother, but in light of what happened in Steamboat Falls, they realize they made a mistake, and now the truth comes out. Jenny is his sister, and he's free to marry Jill.
