Colonel John Hardin and his men are returning from their last adventure with the Corsican mafia when they come upon a squad of marines under attack at a temple. After they rescue them and find out they are guarding a beautiful photojournalist. Also they recover a strange gas mask. Further examination of the mask points to a highly sophisticated mask of Chinese origin. Combined with reports from remote villages in Laos of a highly deadly gas they call sky fire. It points to a Chinese nerve gas being developed for use against US troops in Vietnam.
Hardin is later kidnapped by General Tho the dapper North Vietnamese that Hardin had a run in with the first two books. He wants Hardin to destroy the nerve gas facility across the DMZ. Tho has concerns of growing Chinese influence in the government. He gives detailed plans for the facility. Hardin and his men go off to destroy it. On the way they join up with the marine squad and the photojournalist who are in fact KGB agents. The Soviets are also concerned with growing Chinese influence in Vietnam and join forces.
The final book in the series has it all. Evil Chinese scientists and beautiful KGB agents. Now does it make sense that the Vietnamese would use a massive nerve gas attack. Well that one of the large logic gaps you could fly a 747 through with plenty of wing room to spare. You just have to go with it.
I notice for a Vietnam war series that the North Vietnamese are not really the main enemy in any of the books. Sure they are killed by the boatload but they just seem to be in the way. The real enemy is the corruption in the army. Traitorous generals, rogue redneck special forces with CIA support, Corsican/Sicilian mafia also with some CIA support. Now the last is the evil Chinese and their new nerve gas. I guess this is the Vietnam war through the eyes of an Englishman. It was an enjoyable journey through the pulpy books of yesterday.