Fine PBS drama tells story of Medgar Evers

''Somewhere in Mississippi lives the men who murdered my husband. This is their story,'' is the startling way that one of public broadcasting's finest hours begins. Then the program goes on to tell with great restraint the emotional and inspiring story of the final years in the life of civil rights leader Medgar Evers.

For Us, the Living (PBS, Tuesday, 9-10:30 p.m.) is yet another fine drama in the innovatively varied ''American Playhouse'' series. Despite the fact that its climax is a shattering assassination, the dramatization overflows with the thrill of true righteousness, the nobility of a people demanding justice.

While the fires of frustration were burning all over Jackson, Miss., in the 1960s, the flames began spreading to the rest of America and soon most legal restraints against social justice were eliminated; the beginnings of a change in social attitudes began to make itself apparent. ''For Us, the Living'' is 90 minutes of testimony to the moral strength of a people, but even as much it is a tribute to one man's insistence upon acting out his beliefs.

The simple, straightforward script by Ossie Davis, directed with the same sort of skillful simplicity by Michael Schultz, is based upon a book by Mrs. Medgar Evers. The widow's words and attitudes provide the backbone for the drama. Her heartrending struggle to join her husband in the ambulance after he is shot - only to be held back with the words ''You can't go. They are taking him to a white hospital'' - was, perhaps, the supreme personal indignity. It is a poignant moment in the script which cannot easily be forgotten.

Although the acting in this program is uniformly excellent, ''For Us'' is not a perfect drama. It tends to oversimplify complex situations and complex relationships. Almost all whites in the program are villains; almost all blacks are heroes. But perhaps that is quibbling when one considers the overall effect of the drama, the pinpointing of the ugliness of bigotry especially when contrasted with the grandeur of human dignity.

In the end, Mrs. Evers surveys Jackson, Miss., and the whole South. ''It's better,'' she says. ''It really is better. But the struggle goes on. . . .''

''For Us, the Living'' carries the struggle one giant step further.