Katie organizes a surprise birthday celebration for her boyfriend, Jack, on the 4th of July. As she decorates, she displays her Type-A personality, meticulously trying to keep everything on track. While her parents, Gayle and Gary, along with friends, work on the preparations, she takes a break to pick up the cake. During this journey, she decides to also collect her brother Seth and his young daughter Ella.
Throughout the movie, Katie experiences vivid hallucinations where she sees Seth and herself struggling as their car fills with water. These visions coincide with an audiobook reading about a woman who fails to sail a boat, ultimately leading to her drowning.
Upon arriving at Seth's apartment, she discovers heaps of unopened mail, raising her concerns about his potential heroin relapse. Desperate to help, she drives around, seeking a detox center, only to learn that his previous place won’t accept his health insurance. After sending Seth via taxi to a downtown center, she returns to the party with a slumbering Ella, only to receive heartbreaking news that Seth cannot be admitted.
When Katie finds Seth in a precarious condition, she continues searching for assistance in Los Angeles, missing Jack’s arrival at the party. Seth begs her to procure heroin for pain relief, prompting a trip to a homeless area near Skid Row, where she gets the drugs concealed in six small balloons. Back in the car, Seth urges her to buy a needle from a man under a tarp, but Katie refuses, leading them to a nearby drugstore instead.
At the drugstore, the pharmacist suspects Seth is a heroin user and discreetly gives Katie the bathroom key, where she changes Ella’s diaper. Meanwhile, Seth preps for his injection in a vacant stall. Despite Katie's pleas for him to use cautiously, he seems revitalized afterward. While leaving, Seth jokingly hands Katie back the bathroom key, which unintentionally triggers an alarm at the pharmacy, causing her to flee back to the car.
As they drive back to the celebration, Katie reflects on childhood memories with Seth and they stop at a bakery for the cake. During the ride, Seth assures her that his most recent injection will be the last one.
Once they arrive at the party, Katie’s family and Jack quickly notice Seth’s intoxicated state due to his erratic behavior. They confront her about the events of the night, leaving Katie feeling guilty. Seth abandons Ella in the backyard while he slips out to use, prompting Ella to search for him around the house. Katie ultimately discovers Seth injecting drugs again in her car.
In a moment of tough love, Katie tells her brother that she can no longer support him, insisting that the only way for Seth to truly overcome his addiction is to seek rehabilitation independently. Free from the weight of her brother's struggles and the haunting hallucinations, Katie steps out of the car to admire the fireworks lighting up the night sky.
