After Christine's younger sister passed away suddenly last year, she closed off that chapter, having navigated through the stages of grief. Focused on her role at a Seattle-based social services nonprofit, she prefers not to dwell on the painful past.

However, her routine takes an unexpected turn when Keji, a grad student, reaches out with unsettling news. Walter, the eerie therapist Christine consulted a few times, is gossiping about her and has even shared her phone number with Keji.

As Christine and Keji delve deeper, they uncover a disturbing connection beyond Walter. Keji's family has also experienced a tragedy, raising questions about the strange coincidence that brought them together. Could there be a reason behind Walter's actions, linking the sorrows they've both endured?

In a narrative intertwining loss, friendship, and the challenges of modern adulthood, Christine grapples with unraveling the mystery surrounding her sister's demise. A Sister Ago serves as a poignant reminder that no family is truly "normal," each harboring its unique complexities. Will Christine finally find the courage to confront her past?