A group of determined soldiers, led by Major Baker, is dispatched to investigate an off-limits basement station that has reportedly come under attack. During the assignment, Baker makes contact with his superior, Gen. Slater, as they delve deeper into the mystery surrounding a civilian van that had encroached upon the base. It is soon revealed that this vehicle belongs to the sole attack survivor, Dr. Bradford Crane, a scientist whose harrowing experiences set off a chain of events that would lead to unforeseen chaos. Slater, alarmed by the dire nature of the situation, issues an order to send out two helicopters for further reconnaissance, specifically to scout an ominous black mass nearby—thought to be an unidentified threat—only to discover that it consists of swarming killer bees. Tragically, the helicopters are overwhelmed by these fearsome insects, leading to a catastrophic explosion that results in the loss of the pilots' lives.
In the melee, Crane adamantly insists to Slater that the military base itself is under siege by the very same African killer bees that caused the destruction of the helicopters. Adding credence to his claims, Helena Anderson, a doctor stationed at the base, stands by Crane, asserting that his warnings should not be taken lightly. Meanwhile, in the serene countryside, a horrific attack unfolds as a family becomes the latest victims of a bee onslaught. In a frantic bid for survival, the teenage son narrowly escapes in a Mustang but bears the sting of the bees, crashing into the heart of Marysville just as the town gears up for its annual flower festival. He is subsequently treated by military officials, plagued by visions of colossal bees—a psychological consequence of his painful experience. There, Dr. Walter Krim eventually confirms to Crane something they have dreaded: the war against these bees has indeed commenced.
As chaos brews, Slater faces the raw emotions of a grieving local, Jed Hawkins, who demands to see the remains of his son, a casualty of the relentless bee attacks. Despite the sorrowful atmosphere surrounding the base, Slater devises a plan to airdrop poison onto the menacing swarm; however, Crane cautions him, deliberating over the environmental ramifications of such a violent method. Meanwhile, the stung teenager and his friends set out to locate the hive for a firebombing, a reckless move that only serves to further provoke the brutal insects. The angry swarm then zeroes in on Marysville, wreaking havoc and claiming hundreds of lives—including innocent children attending local schools.
Amidst this growing crisis, Crane and Helena seek refuge in a diner alongside a pregnant waitress named Rita. Observing the events unfold with a mix of trepidation and urgency is reporter Anne McGregor, who remains in the safety of her news van, eager for thrilling footage of the siege. In the wake of yet another brutal attack, Slater proposes an evacuation for the Marysville residents via train. However, calamity strikes as the bees target the evacuees, resulting in more tragic deaths—including those entwined in a heartbreaking love triangle: schoolteacher Maureen Scheuster, retiree Felix Austin, and the town's Mayor Clarence Tuttle, who also manages the local drug store.
In a twist of fate, Rita, confined to a hospital bed, welcomes her child into the world while finding unexpected romance with the attending doctor. As the relentless swarm sets its sights on Houston, Crane resorts to dropping eco-friendly bombs, hoping to deter the swarm by disrupting their senses. Unfortunately, this plan yields tragic consequences—resulting in the death of the young boy who initially unleashed this terror upon Marysville, igniting fury within Helena over the senseless loss of life. In a desperate attempt to combat the outbreak, Dr. Krim administers an experimental bee venom antidote, only to succumb to the fatal side effects of his trial.
Amid the turmoil, the manager of the local nuclear power plant, Dr. Andrews, remains obstinately convinced that his facility can withstand any ensuing attacks, dismissing the warnings issued by Dr. Hubbard. Unfortunately, he is proven gravely mistaken as the alarm blares and the bees invade, claiming the lives of both Andrews and Hubbard and obliterating the plant, resulting in catastrophic devastation for the surrounding area.
In their fight for survival, Crane and Slater review footage from the original bee invasion and deduce that their alarm system may have inadvertently lured the swarm toward the base. As the bees mount another offensive, Slater bravely employs a flame-thrower to facilitate Crane and Helena's escape. Ultimately, they manage to entice the bees to the ocean using sound-altered helicopters, where they douse the waters with oil and ignite the swarm in flames. However, as Crane contemplates the outcome, he reflects, "if we use our time wisely, the world just might survive," pondering whether their victory is lasting or merely temporary.
