As the reality of Lorna Cole's (Rene Russo) pregnancy with LAPD Sergeant Martin Riggs' (Mel Gibson) child sinks in, the couple begins to contemplate what their future holds. At the same time, fellow officer LAPD Sergeant Roger Murtaugh's (Danny Glover) daughter Rianne finds herself in a delicate situation, as her pregnancy remains a closely guarded secret due to her relationship with Detective Lee Butters. The pressure mounts when the Police Chief decides to promote both Riggs and Murtaugh to captains, despite concerns from the department's insurance provider regarding their previous actions as sergeants, setting off a series of events that challenged their newly acquired authority.
Amidst these personal struggles, the officers uncover a Chinese immigrant smuggling operation while investigating an oceangoing vessel that has been intentionally run aground. Murtaugh's empathetic character shines through when he offers refuge to the Hong family, who are in fear of deportation from the United States. The desperation of the Hongs resonates deeply with Riggs, Cole, and Murtaugh's families, fostering strong emotional connections through shared values and a commitment to justice.
An important lead surfaces when Grandfather Hong reveals his search for a talented engraver, his uncle, who financed their journey to America. At the same time, Lorna's sensitivity towards Rianne's circumstances compels Riggs to uncover the concealed truth of her pregnancy, a secret that stretches the bonds of their partnership.
The investigation heats up as the officers confront "Uncle" Benny Chan, a formidable crime boss operating from a Chinatown restaurant. Introducing high-ranking Triad negotiator Wah Sing Ku adds a layer of intensity to the impending confrontation. Riggs and Murtaugh's inability to establish probable cause leads Chan to force them out of his establishment, escalating tensions to a dramatic climax.
The situation deteriorates when Riggs' attempt to apprehend the escaped ship captain fails, resulting in Ku punishing him brutally as a warning. The Triad's quest to eliminate anyone exposing their counterfeiting operation puts Murtaugh's family at significant risk. In a gripping twist, a daring rescue orchestrated by Hong's resourceful grandson, Ping, brings some of the captives to safety just in time. Although Riggs and Murtaugh manage to thwart some of the Triad's pursuers, Ku narrowly escapes, determined to eliminate all who threaten his organization.
The climax settles upon a warehouse where Mr. Hong is coerced into engraving plates for Ku's illegal business, ensuring his family's safe passage in return. In a heart-wrenching moment, Ku brutally executes Hong in front of his uncle to secure his continued compliance, illustrating the lengths to which he will go to uphold his nefarious plans.
As chaos ensues, police officers initiate an odd diversion at Chan's dentist office, attempting to extract crucial intel while under the influence of laughing gas. Yet, in a chaotic turn of events, Riggs and Butters accidentally divulge a secret to Murtaugh during this unusual interrogation, setting off a chain of unforeseen consequences. Eventually, Detective Ng, a specialist in cases involving the Chinese government, uncovers that Ku is negotiating with a corrupt Chinese general for the release of the Four Fathers, which includes Ku's own brother. This understanding leads the police to a warehouse, uncovering the gruesome fate of Mr. Hong and his uncle, both murdered as soon as they were deemed useless.
With urgency mounting, the officers race against the clock to prevent the exchange between Ku and the general, ultimately exposing the counterfeit scheme. In response to this betrayal, the enraged general seeks revenge on the Four Fathers before being mercilessly killed by the Triads in an explosive firefight. As the dust settles, Murtaugh's friend Butters is injured while saving him from a Triad attack.
Unyielding, Ku attempts to flee with his brother, only to witness him fall victim to Murtaugh's pursuit. Riggs and Murtaugh engage in a visceral confrontation with Ku on a pier. In a moment of desperation, Murtaugh pierces Ku with rebar, yet the vicious criminal manages to knock him out. The collapsing pier sends both Riggs and Ku into the water, enhancing the suspense.
Riggs seizes an opportunity to retrieve a Kalashnikov assault rifle and delivers the final blow to Ku, while Murtaugh fights against the odds to rescue Riggs from a crushing concrete slab that trapped him beneath the water.
After the battle has concluded, Riggs visits his late wife's grave, seeking solace as he prepares for his uncertain future with Lorna. In a touching moment, Getz shares a story from his childhood that sheds light on their friendship and provides Riggs with a fresh perspective.
As they await the arrival of Lorna's child, Riggs and Murtaugh learn that Butters will soon join their family. With the city secured and the Hong family granted asylum, Murphy reinstates Riggs and Murtaugh's ranks as sergeants, recognizing their relentless commitment to restoring peace in their community.
