If this episode seems familiar, it may be because you saw the 1975 episode "The Hostage"--which has a plot very, very similar to "King of the Hill". Both are about military men who are suffering from Post-traumatic Stress Disorder and literally think they are back in action--barricading themselves and innocent people with them in an effort to "protect" themselves and their friends from attack. Of course there are many differences in the two episodes, though there's little doubt this later episode was strongly inspired by "King of the Hill".
The episode begins with Yaphet Kotto being brought into a hospital emergency room. It seems that he's a marine on leave from Vietnam and was accidentally hit on the head with a baseball bat. However, when he revives, this shock is enough to convince him that he's back in Vietnam and he's surrounded by the enemy. The problem is that he steals a police officer's gun and shoots several people--including Danny Williams. But, because the guy's head is so scrambled, he thinks Danny is his sergeant back in Vietnam and he's determined to barricade the both of them in the hospital room--shooting at anyone who tries to approach to stop him or help good old Danno. So how do they get Kotto to relent and let doctors get to Danny before it's too late?
Overall, this is a highly enjoyable episode. Much of this is due to Kotto's excellent performance--it was rather amazing. The only problem is, that the episode seems amazingly improbable. So, provided you don't think too much, this is a great episode.
By the way, look at Kotto's IMDb biography--he's an interesting man with an interesting background to say the least.