Mycroft sat leisurely in his home cinema, watching an old film he could recite by heart. Only at such times could he briefly forget annoying political affairs and his troublesome brother. Suddenly, childhood footage flashed on the screen: his father, mother, and brother Sherlock, followed by the words "I'm back." He jumped from his seat, suddenly feeling the surroundings were eerie. Leaving the room, he grabbed his umbrella leaning by the door. With a quick pull, he revealed a sword hidden within the shaft. A little girl ran wildly through the dark rooms, always just out of Mycroft's reach. Forcing calm, his first thought was that his sister, Eurus, had escaped. Muttering "impossible," he refused to believe she could break out of "hell." At this tense moment, Sherlock walked in from behind a door. A whistle sounded, the lights blazed on, and John also revealed himself; they had been behind the prank.
John had woken in the therapist's office to find Eurus gone. He didn't know why she spared him, or even if she truly was a Holmes sibling. Sherlock had orchestrated this to test Mycroft's reaction. From it, Sherlock deduced three things: 1) He did have a sister named Eurus; Mycroft had her locked away in a state institution; 3) Mycroft was terrified of her.
The next day, Mycroft stood in the sitting room of 221B Baker Street. Mrs. Hudson, unable to resist gossip, tried to eavesdrop, but this was both a state secret and a family matter. Sherlock sent the disappointed landlady away but insisted John stay; he wouldn't take the case without his partner. Reluctantly, Mycroft sat down and explained everything.
It all began at the family's ancestral home, Musgrave Hall, where young Sherlock lived with his parents, his seven-years-older brother Mycroft, and his one-year-younger sister Eurus. Near the estate stood a row of a dozen headstones over empty graves, bearing false names and dates. Young Sherlock, obsessed with pirate games, ignored the mystery of the stones and the sister who constantly followed him. Eurus, growing up without playmates, developed cognitive abilities far beyond normal. Perhaps out of childish jealousy, she hid Sherlock's dog, "Redbeard," to punish his neglect. To find Redbeard, Sherlock had to solve a nursery rhyme she composed. As Eurus sang it repeatedly, Sherlock searched the grounds in vain until she began singing of "drowned Redbeard." The outcome was grim. Afterwards, Eurus spiraled further, burning down the hall. Authorities took her away. The trauma fundamentally changed Sherlock; he never spoke of Redbeard again and suppressed all memory of his sister.
For years, Mycroft never spoke of it. Ordinary asylums couldn't hold a mind like Eurus's. Under Mycroft's arrangement, he personally consigned her to "hell"—the maximum-security Sherrinford prison on a remote island. He told their parents she died in another fire. He never imagined she'd reappear.
Just then, Eurus's voice filled the room. A drone rose from a kitchen corner, playing the unsolved nursery rhyme, a grenade mounted on top. Mycroft recognized it as a type he'd authorized for military use—motion-sensitive. Flames and explosion erupted from 221B's windows. Sherlock and John leapt out just in time; Mycroft's fate, nearest the door, was unclear.
Miraculously, no one was seriously hurt. The incident only intensified Sherlock's desire to meet his sister. Mycroft decided to test Sherrinford's security. To approach the island, Sherlock hijacked a fishing boat. The suspicious vessel was quickly intercepted. The Governor recognized Dr. John Watson and assumed the old fisherman was Sherlock in disguise. He ordered a guard to reinforce the watch on Eurus's cell.
The Governor was only half right. The "fisherman" was Mycroft. Sherlock, disguised as a guard, had "escorted" them inside and was now boldly heading to see his sister under orders.
Eurus sat in her fortified glass cell, playing violin with her back to the entrance. She seemed to know Sherlock was watching. Sherlock felt little connection, demanding to know how she left and returned. Instead of answering, she exploited his desperation, psychologically compelling him to approach the "glass" wall. To his shock, there was no glass. Eurus leaped through the frame, choking him. Guards rushed in—not to save Sherlock, but to protect Eurus. Sherlock was stunned.
John discovered the reason. Watching psychiatric evaluation tapes in the Governor's office, he saw how Eurus manipulated every expert, driving one to murder his family and commit suicide. Mycroft knew she had this ability to find and exploit weaknesses. The final tape featured the Governor himself; Eurus controlled him, and by extension, the entire prison. The Governor, exposed, sounded the alarm. Mycroft and John were captured. Eurus wasn't just controlling individuals; she controlled Sherrinford.
Her childhood dream was to have her brothers play her game. The plan began five years ago, with Mycroft's help. Needing her intellect, he granted requests: first a violin, then, five Christmases ago, five unsupervised minutes with Moriarty. No one knew what was said, but Moriarty's subsequent actions were likely orchestrated by Eurus. Mycroft and Sherlock had walked into a trap.
The game began. Sherlock, Mycroft, John, and the Governor were locked in a cell. A distorted voice, a girl on a plane where everyone else slept, pleaded for help over the intercom. Sherlock tried to assist, but Eurus cut the link. To continue, they had to solve the first puzzle: a pistol in the cell. Sherlock had to order Mycroft or John to kill the Governor, or Eurus would kill the Governor's wife, shown on screen.
The Governor, seeing his wife bound, begged Sherlock to do it. Mycroft refused outright. John thought he could, but his morals prevented him. In despair, the Governor seized the gun and killed himself. It didn't save his wife; Eurus shot her anyway. She wanted to prove morality was useless, costing innocent lives. Having made her point, she ordered Sherlock to the next cell.
For motivation, she reconnected the "girl on the plane." A brief exchange confirmed it was night, eliminating half the globe. The second game was a deduction: a man shot from 300 yards with a rifle on a table. Three suspects were suspended outside a window over a shark-infested sea. Sherlock had to identify the real killer. John identified the rifle as old, with heavy recoil, ruling out one suspect (who'd have injuries from the scope). Another had a drinker's nose and tremors, making a 300-yard shot impossible. The remaining man was the murderer.
Normally, police would investigate based on Sherlock's deduction. Here, a word meant death. Hesitating briefly for the plane's passengers, Sherlock gave the verdict. He was correct, but Eurus didn't care about guilt; all three men fell to their deaths. Innocent or guilty meant nothing to her.
Before the next game, Sherlock learned the plane was heading for a lit-up city. The next puzzle: a coffin. From its size and workmanship, Sherlock deduced it was for a woman of modest means. A plaque read "I love you," implying a close connection to Sherlock. Ruling out the luxurious Irene Adler, that left Molly Hooper, who had long loved him.
A screen showed Molly in her kitchen. Eurus had planted a bomb there. Sherlock had three minutes to make Molly say "I love you," or die. Knowing Sherlock's history of hurting Molly, John and Mycroft were doubtful. After a rejected call, Sherlock begged on the second try, even saying the words first. Molly, finally said "I love you" with two seconds left.
Molly spoke from true feeling, knowing a relationship was impossible, causing her great pain. Sherlock "won," but Eurus mocked him; there was no bomb. Sherlock had only retraumatized Molly. Enraged, Sherlock smashed the coffin.
The game continued. Sherlock and John encouraged each other, watched by Mycroft. The fourth game was crueler: Sherlock's gun had one bullet. He must choose who lived—John or Mycroft—shooting the other. Mycroft shed his, insisting Sherlock shoot the "idiot" John. John, knowing future games required intellect he lacked, accepted his fate. But Sherlock saw through Mycroft's act; his brother was sacrificing himself, blaming himself for the five-minute meeting with Moriarty. Sherlock refused this outcome, modeled on the Governor's choice, and turned the gun on himself.
Eurus hadn't anticipated this. She fired a sedative dart, knocking Sherlock unconscious.
He awoke hours later to the girl's voice. Moonlight shone through a skylight. The "girl" had found the unconscious pilot. Through her description—a river and a Ferris wheel ("The London Eye")—Sherlock realized the plane was over the Thames. He felt powerless to guide her to the radio.
Looking around his concrete cell, he called for Mycroft. There was no reply, but John's voice came through his earpiece. John was in a well, chained, finding small bones—animal or human. A "Redbeard" food bowl under a table and a draft made Sherlock realize they weren't at Sherrinford. He kicked down the flimsy wall; outside was the ruined Musgrave Hall.
His first unsolved case—finding "Redbeard"—was his last. Failure meant John's death. As Sherlock wrestled with the rhyme, John found more bones—a child's skull. Sherlock remembered his father was allergic to dogs; there never was a dog. Mycroft had invented "Redbeard" the pet to hide the truth.
Repressed memories surfaced. "Redbeard" was Sherlock's best friend, Victor Trevor. Sherlock and Victor, playing pirates, ignored Eurus. After Eurus killed Victor, Sherlock altered his memory. Now, to save his best friend, Sherlock's mind raced, solving the rhyme's code using the dates on the fake graves.
The decoded message revealed a childhood cry for help: young Eurus, feeling lost, begged her brother to come to her room and save her soul.
Sherlock finally understood. There was no crashing plane. The "girl" was Eurus herself as a child, playing with a toy plane, high above and lost, with nowhere to land until her fuel—her soul—ran out. He ran to Eurus's old room in the manor, where she waited for him to solve the final puzzle and redeem her lost soul.
Sherlock rescued John from the well and found Mycroft in a mock-up of Eurus's cell. Eurus was returned to Sherrinford under Lestrade's guard. Mycroft faced his parents' wrath. Sherlock couldn't bring Eurus home, but he visited her with his family. Afterwards, Eurus refused to speak, so Sherlock communicated with her through violin music, providing the childhood connection she lacked.
221B Baker Street was restored, complete with bullet holes. Lestrade and Molly became regular visitors. Between caring for Rosamund, John rejoined Sherlock's adventurous life. As Mary had wished, Sherlock and John supported each other, bringing a glimmer of hope to a world of crime.
