From the perspective of Fang Hongye, the film unfolds a tense series of murders at the Shen's Family Fort, pulling an outsider into a tangled web of secrets until he becomes the pivotal observer and participant in the escalating mystery. The atmosphere is dense with suspicion as each new clue tests his resolve and pulls him deeper into a case that refuses to stay neatly contained.
The story pivots around the enigmatic "Hongye's Notebook," a hand-written record of unusual happenings in the martial arts world, or Jianghu, that signals a pattern behind the killings. Eight pages of the notebook, later revealed to be forged, describe a series of murders committed by a mysterious force described as a killer butterfly. These butterfly-linked deaths connect old tomb robberies to a current wave of bloodshed, suggesting that past rivalries are resurfacing in lethal and unpredictable ways. As decades pass, another tomb raider dies in a eerily similar fashion, prompting Shen Qing to summon Tian Feng of the infamous "Seventy-two Smokes" gang and others to investigate alongside the fort’s own authorities, hoping to break the pattern once and for all.
Upon their arrival, the investigation intensifies the danger. The fort owner himself, Shen Qing, becomes a target as the butterfly murders continue to claim lives and escalate panic across the Jianghu. Shen Qing’s wife urges patience, insisting they await the arrival of three brothers known as the "Four Phases of Heavenly Thunder" before any decisive action is taken. The tension rises as suspicion turns inward and the line between ally and adversary blurs.
A mute servant girl named Ah-Zhi becomes a crucial guide, leading Hongye to a hidden gunpowder storage room and raising questions about what else is being hidden in the fort. Just as the investigators gather, the long-awaited trio finally arrives, shifting the balance of power and heightening the sense of peril. The plot takes a startling turn when Shen Qing himself dies in a figure of iron-clad violence, the mystery deepened by the realization that this seemingly straightforward death is tied to the larger riddle.
The truth that emerges reframes everything: six years earlier, Yu Zhen—revealed as the deceased fourth brother of the Four Phases—founded the Shen's Family Fort as a covert workshop to develop an alloy for powerful gun barrels. With the weapon completed, Yu Zhen engineered the butterfly murders and forged the eight pages of Hongye’s Notebook to eliminate rivals and seize control of the martial arts world, all while manipulating Tian Feng into aiding him unknowingly. The entire scheme is a masterclass in manipulation, using forged history and staged crises to consolidate power.
In the climactic denouement, Hongye exposes the truth, leaving the fort behind him as the conflict reaches its brutal end. Yu Zhen and Tian Feng, along with others, perish in a final battle that destroys both the fort and the weapon they sought to wield. The conflagration marks the collapse of a carefully crafted illusion and the end of a dangerous bid for dominance—an ending that resolves the murders but leaves the world of Jianghu forever altered by the revelations unearthed within the fort’s walls.
