Outwardly Elise Dutton would seem to have her life just the way she wants it. She is the efficient and well-liked coordinator of student travel with a travel agency and is competent and good at her job. She appears popular and has appreciable social skills, and also seems happy and confident. However the outward picture of Elise is very different to the inner one; years before, Elise was driving her daughter, Hannah, to daycare when her office called. They could not find the contact details for a client that had been mis-filed in the office Rolodex and needed Elise's help. Intending to run into work for just a couple of minutes, Elise left Hannah in the car. She became so sidetracked that by the time she returned to the car, Hannah had died from the extreme heat. The death was investigated and a kind assistant prosecutor discovered a syndrome that involved women compartmentalizing their lives and demands on them to such a degree that they literally forgot what they had been doing moments before. He believed this had occurred in Elise's case and Hannah's death was ruled an accident. Elise became a pariah in public and was ripped apart by the media but even though public anger eventually died down, Elise's anger with herself did not.
Every day, Elise would eat the same salad for lunch sitting in the same seat at the food court in the office building where she worked. One day she thought she saw a handsome man looking over at her but dismissed the notion. A few days later the man approached her and introduced himself. Nick was a successful attorney, a partner at a law firm also located in the same building. He joined her for lunch - and he came with an interesting proposition. It is the holiday season and with that comes seasonal professional obligations. Nick does not want any personal drama and so proposes that he and Elise agree to act as each other's dates as they negotiate the tricky holiday season. Initially Elise says no but after a great deal of thought she changes her mind and agrees. Nick has drawn up a contract which they call The Mistletoe Promise. They agree to eat lunch together every day, to attend their respective work parties together, to hold hands in public and also to go out on one additional date each week, alternating Nick's choice of venue or activity with Elise's. Nick will pay for everything.
The contract starts with Nick sending flowers to Elise at work. Elise's co-worker, Zoey, assumes they are for her as she has multiple boyfriends and nobody has ever sent flowers to the office for Elise before. Elise tells Nock this over lunch, saying that flowers and attention are for beautiful girls like Zoey,not for girls like her. Nick then adds "believing you are beautiful" to the terms of the contract. On day four he surpasses himself and sends twelve dozen red roses to Elise at work. The following week he sends her an exquisite antique mirror. It has been so long since Nick has had anyone to spoil that he is getting more pleasure from showering Elise with gifts than she is from being the lucky recipient. Elise's coworkers believe that her mystery man is sending so many gifts to compensate for the fact he is most likely fat, short and bald. Overhearing this, Elise is affronted on Nick's behalf and asks Nick to meet her at her office to take her to lunch rather than meeting in the food court as usual. Zoey is stunned to see how handsome Nick is.
Elise chooses The Messiah Sing-Along as their date and the two become part of a huge pick-up choir singing the music of Handel. Afterwards they go to eat and Elise opens up about her whole life - apart from Hannah. Although they talk easily and Nick is totally non-judgmental, Elise realizes that although he knows a great deal about her she still knows hardly anything about him.
On Thanksgiving Eve, Nick goes to Eluse's house for dinner and watches her make pies, the one positive thing her mother passed onto her and a talent Elise is renowned for. As Nick tells her one of his favorite things about his childhood Thanksgivings was the pie crust cookies his mother would make, Elise shows him the batch of pie crust cookies she has already laid out on the baking sheet. They eat them together whilst watching "It's A Wonderful Life" on the television.
Nick's close friends, Sharon and Scott Heitesman, always host Nick for Thanksgiving and this year Elise, and her pies, join them too. It is a relaxed, enjoyable day and Elise is shocked when Scott tells her he has never seen Nick so happy and that he is clearly madly in love with her.
The following morning Nick calls Elise early to go Black Friday shopping. Their ability to enjoy everyday things with each other pleases Elise. Dan, her ex-husband, is most definitely not pleased that she is happy and in a new relationship. He is very jealous. He tells her that his wife is cheating on him and that he wants to try again with Elise. He also says that she should take him up in the opportunity because nobody else will have her when they find out what happened to Hannah. Elise orders him to leave.
Elise invites Nick to her office's holiday party which is held at her boss, Mark's, house. It is a casual affair with an over abundance of beer and not a great deal of food which makes everybody tend towards being drunk. An inebriated Zoey flirts wildly with Nick, who does not respond, but Elise is very upset with Zoey. At work, Zoey is apologetic and to show how sorry she is offers to style Elise for Nick's firm's holiday party which will be a far more elaborate affair than theirs. Surprisingly Elise warms to Zoey who is a sensitive, thoughtful girl underneath the constant flirting and apparent man-madness. Zoey confides that she wishes someone would love her the way Nick loves Elise. Elise looks absolutely stunning and Nick obviously loves introducing her to his co-workers.
Nick's mood changes abruptly on December 6th. Elise cannot reach him emotionally and he seems very depressed. He tells her that this passes and that he feels this way every December 6th but does not elaborate further. After a very worrying weekend Elise is relieved that he is back to normal on Monday and delighted that he invites her to accompany him on a business trip to New York. Nick has booked a luxury hotel on Sixth Avenue, and they see The Rockettes perform at Radio City Music Hall. Nick asks her to pick up a package for him at Tiffany's. Wholst there she selects a beautiful and expensive Mont Blanc fountain pen for Nick, happy to be able to give him a gift this time. At dinner they confess their love for each other. Elise remarks that if Nock really knew her he would not be able to love her; he asks if that is because of Hannah. Nick then tells Elise that he was the gentle, patient assistant District Attorney who had interviewed her. Elise is horrified that her past has come with her into her new relationship. She eaves the restaurant and flies home leaving Nick in the city.
On Christmas Eve, Dan visits again triumphantly slapping a newspaper article onto the counter and telling her that her new love is a convicted felon. When he was sixteen it appears Nick had driven home from a party whilst over the limit, causing an accident that killed three members of the same family. He tells Elise that she and Nick are a perfect match because they are both killers. Furious, Elise throws him out. She drives to Nick's house to apologize realizing that he can love her because he knows exactly how she feels. He tells her the whole story; how, on December 6th, as a teen he had driven his drink friend home from a party not realizing that the first couple of beers he ever drank would impair his driving abilities. He caused the a consent and was sentenced to serve time at a juvenile facility. When he was released his family had disintegrated and disowned him. He admits the contract was the safest way to find out of these two damaged people could truly love each other.
Elise and Nick get married in Italy the following year with his estranged sister in attendance, wanting to connect with him again. Their child is born a year later, in the same month that Hannah was born.