During the Vietnam war, the American army forms a special unit called "tunnel rats" to go down into the elaborate system of underground tunnels constructed by the Vietcong, kill anyone they find in it and destroy the system.

An outbreak of bombings, assassinations, and massacres by an extremist, Marxist terrorist group, alarms the international community who calls for help - the ultimate solution - the Trident Force.

After their mission is overrun by North Vietnamese forces, a missionary couple, their Australian guide, a South Vietnamese major, a brothel owner and her daughter all get in the mission's battered school bus and try to make their way to safety. Soon after leaving, however, their bus is commandeered by three US Marines trying to get back to their base at De Nang. To reach their destination, they must fight through most of the North Vietnamese army.