In 1947, the once-renowned detective Sherlock Holmes, now 93 years old, resides in a tranquil farmhouse in Sussex alongside his widowed housekeeper, Mrs. Munro, and her young son Roger. Fresh from a journey to Hiroshima, Holmes has begun experimenting with a prickly ash plant he obtained to aid his declining memory. He is particularly troubled by the portrayal of his last case, The Adventure of the Dove Grey Glove, written by his former companion, Dr. Watson. Driven to pen his own narrative, he finds himself struggling to recollect the events.
Holmes gradually forms a bond with Roger, entrusting him with the care of the bees in the farmhouse's apiary. As they spend more time together, Holmes is enchanted by Roger's curiosity and mind, leading to a burgeoning paternal affection. Through their interactions, Roger’s inquisitiveness prompts Holmes to uncover memories of his past case, revealed in flashbacks that hint at a significant failure leading to his retirement from detective work.
The narrative shifts back nearly 30 years, to a time shortly after the First World War. Watson had married and moved from Baker Street, leaving Holmes to grapple with a case regarding Thomas Kelmot. Thomas sought Holmes’ help to understand why his wife, Ann—played by Laura Linney—had grown estranged after enduring two miscarriages. Holmes followed Ann through London and suspected her of plotting to murder her husband as he witnessed her forging cheques, purchasing poison, and planning her demise. However, Holmes discerned her true intentions: she aimed to secure gravestones for herself and her lost children, followed by her own suicide.
In their fateful meeting, Ann confided in Holmes, expressing a mutual sense of isolation. Though tempted to share the burden of loneliness, Holmes ultimately urged her to return to Thomas. Gratefully, Ann poured the poison onto the ground, yet tragically, in later news, Holmes discovered that Ann had ended her life under a train. Consumed by guilt, he withdrew from the world, spiraling into despair—the narrative adapted by Watson into a more uplifting tale.
The flashbacks continue with Holmes’ recent visit to Japan, where he encountered a man named Tamiki Umezaki. Initially perceived as an admirer, Umezaki confronted Holmes about his father's disappearance during a business trip to England, believing Holmes had influenced him to abandon his family. Disappointed by Holmes' cold response, Umezaki felt robbed of a connection he had always cherished.
Back in the present, Mrs. Munro grows increasingly frustrated with the demands of caring for Holmes, especially as he becomes more reliant on her assistance. Tension ensues between her and Roger, who harbors dreams beyond their modest life. When Mrs. Munro secures employment at a hotel in Portsmouth, Roger dreads the idea of leaving Holmes behind. One day, Holmes discovers Roger unconscious after a reckless encounter with wasps in the garden while attempting to protect the apiary. The incident leads to Mrs. Munro blaming Holmes for being self-centered. However, they work together to resolve the issue and rush Roger to the hospital, where he begins to recover.
Amidst this, Holmes confesses to Mrs. Munro that he was too scared and self-involved to offer Ann the comfort she craved, expressing a desire for her and Roger to remain a part of his life. He assures them they will inherit his estate upon his passing. Following this momentous revelation, Holmes composes a letter to Umezaki, portraying his father in a noble light as a man who valiantly served the British Empire. As Roger helps teach his mother the art of beekeeping, Holmes finds solace in enacting a legacy reminiscent of his experience in Hiroshima—creating a stone circle to remember those he has lost throughout the years.
