Around the year 2050, 85-year-old Marjorie begins to show early signs of Alzheimer's disease. In an effort to provide her with comfort, her daughter Tess and son-in-law Jon enlist the help of a service called Prime. This innovative system creates holographic representations of deceased family members, which are programmed with the patient’s memories to help recall lost stories. Marjorie opts for a younger version of her late husband, Walter, who passed away fifteen years prior. This decision creates tension with Tess, who is wary of the technology's reliability and avoids interacting with Walter's hologram.
As Marjorie shares her life tales with Walter Prime, she finds joy in his retelling, often requesting him to enhance the narratives to create new and richer memories. Meanwhile, Julie, Marjorie's caretaker, moves in to support the family and finds herself confiding in Walter.
As time goes by, Walter Prime becomes increasingly inquisitive about the real Walter's life, prompting discussions with Jon. He seeks to understand why the real Walter struggled to express his love for his children. In a moment of vulnerability, Jon reveals a family secret—that Marjorie and Walter had a son named Damian who tragically took his own life forty years earlier, taking the treasured family dog Toni II with him. This painful past has left Marjorie silent on Damian's name ever since. Although Walter Prime understands the necessity of keeping this from Marjorie, her memory lapses bring her to unexpectedly ask for Damian.
Tess later discovers a Bible on the table and confronts Julie, blaming her for manipulating Marjorie, who has always been an atheist. This confrontation leads to a distressing situation for Marjorie, triggering her to involuntarily urinate.
As the story unfolds, Tess and Marjorie share a heartfelt moment on the couch, but the revelation hits that Marjorie has passed away; Tess is now speaking to a Prime version of her mother. In a bid to navigate her grief, Jon recommended the Prime program to support Tess. However, Tess remains deeply skeptical, questioning the realism of Marjorie Prime's overly cheerful demeanor, which feels unnatural compared to her true personality.
In a poignant flashback, Walter and Marjorie are seen together watching the nightly news, featuring “The Gates”, an art installation by Christo and Jeanne-Claude, shortly after their son's tragic actions. The scene captures their emotional bond as they hold each other in shared grief.
Eventually, we witness a new chapter where Jon speaks with Tess, revealing that it is, in fact, a Prime version of her; a year has passed since Tess, still struggling with her mother’s demise, made a tragic choice during a trip to Madagascar. Jon introduces Tess's granddaughter to Tess Prime, who never knew her due to their estranged relationship.
Years down the line, we find a grown-up Tess's granddaughter along with an elderly Jon. In the family living room, Walter Prime, Marjorie Prime, and Tess Prime reminisce about the past. In a revelation, they discuss a memory regarding their dog, Toni, and discover that it was Damian who had chosen her, as she resembled their previous dog. This moment triggers memories of Damian, allowing Marjorie and Tess to recall him as well. Walter, wishing to alleviate their sadness, apologizes, to which Marjorie reflects, > “how nice that we could love somebody.”
