Kate Hannah, portrayed by Mary Elizabeth Winstead, is a passionate elementary school teacher who endures a rough start to her day when she vomits in front of her class. In a moment of panic, when a curious student asks if she's pregnant, she impulsively says yes, a claim she then feels compelled to maintain when questioned by the school principal, Mrs. Barnes, played by Megan Mullally. Her substitute teacher, Dave, depicted by Nick Offerman, has seen her drinking in her car and warns her about the risks to her imaginary baby. Eventually, Kate confesses to Dave that she isn't pregnant but was, in fact, hungover, prompting him to promise to keep her secret.
Later that evening, she attends a party with her husband Charlie, played by Aaron Paul, and his brother Owen, portrayed by Kyle Gallner. Amid heavy drinking, Kate encounters a woman who solicits a ride home. Their night escalates when Kate tries crack for the first time, leading to a reckless escapade that results in both women passing out on the street.
Upon awakening alone the next morning, disoriented and unsure of her surroundings, Kate eventually finds her way home to a slumbering Charlie. They decide to head out for drinks at a club, where Kate shares her wish to achieve sobriety, only to be met with Charlie's lighthearted dismissal. Their night includes cycling and croquet combined with alcohol, leading to a moment of intimacy before Charlie succumbs to exhaustion. Driven by her desire for alcohol, Kate tries to buy wine late at night but faces a lock on the restroom and ends up urinating on the floor in frustration. Ultimately, she steals a bottle of wine before realizing she's passed out yet again in an unfamiliar place.
Back at work, it's revealed that her colleague Dave is a recovering alcoholic, and he invites Kate to an Alcoholics Anonymous (AA) meeting. There, she bonds with Jenny, played by Octavia Spencer, a woman who has passionately embraced cooking over drinking. Choosing Jenny as her sponsor, Kate begins her journey toward sobriety, supported by the group, but finds it increasingly challenging to resist the temptations of her drinking environment.
One night, after Dave drives her home, he expresses his feelings for Kate and makes an inappropriate comment that deeply unsettles her. Upset, she confides in Charlie about their fading communication and her desire to reconnect with her estranged mother, Rochelle, played by Mary Kay Place. They travel to Lake Arrowhead where Kate's mother lives, discussing AA meetings while Rochelle remains doubtful about her daughter's commitment to sobriety.
In a surprising twist, Kate is hosted a baby shower by her co-workers without her prior knowledge. Tensions flare when Dave apologizes for his previous remarks, only to find himself berated by Kate for their disrespectful implications. At Owen's birthday party, Kate is infuriated to discover Charlie revealed her past drug use to Owen. As the night unfolds, Kate continues to struggle, lie to her students about her weight gain, and ultimately decides she must confess her deception to Mrs. Barnes.
That decision leads to her termination from the school. Frustrated, Kate spirals and relapses. She argues with Charlie, blaming him for his lack of support during her struggles. Despite the turmoil, Kate shares a poignant speech at her AA meeting, celebrating a year of sobriety and reflecting on her emotional journey. Meanwhile, Charlie grapples with his own grief in a bar, realizing he can't escape his feelings for Kate.
Their lives intertwine again when Charlie calls to recount a drunken incident, prompting a reunion filled with familiar feelings as they play croquet. Charlie expresses he hasn't found anyone like Kate since their separation and admits he misses her. He proposes the idea of moving back in together if he attends AA meetings, but Kate stands firm, asserting he must seek sobriety for himself first. As their conversation unfolds, she wrestles with the hope for the future, leaving their relationship's resolution hanging in the balance as the film closes, open to interpretation.
