Two Los Angeles police detectives with contrasting personalities are reluctantly paired together to solve a case. The elder detective, Roger Murtaugh, is a family man and by-the-book cop on the verge of retirement. He is meticulous and cautious in his approach to law enforcement. Opposite him is Martin Riggs, a young detective with a death wish due to his wife's recent passing. Riggs is impulsive, unpredictable, and highly skilled in combat, making him a "lethal weapon" but also a risk to both himself and those around him. Their partnership puts them both on edge, leading to tension and conflict as they strive to understand each other's methods.

As the two detectives delve into their assignment, they uncover a drug-smuggling ring with ex-military operatives involved. The investigation quickly turns deadly when they come across evidence that brings them too close to the truth. They are forced to navigate a series of dangerous situations, and in doing so, they form a tentative bond forged under fire. Together, they face off against a ruthless and well-organized adversary who will stop at nothing to protect their criminal operations. This assignment tests the limits of their resolve, their courage, and ultimately, it either has to break or solidify their partnership.