The Army won't protect the guys stringing up telegraph lines because that's Indian territory by treaty. Announcing that any treaty that favors Indians is no good, they call in Eddie Dean. Dean and Marshall Lash Larue discover a crooked town that's killing the Buffalo to starve the Indians, so Dean and Larue promise to investigate, which is fine by then.
The version I looked at was the shortened 1948 reissue, which may go a way towards explaining why Eddie gets one song in the middle, and the. chorus as everyone rides off at the end. Otherwise it's a fairly standard B western, with some poorly executed fight choreography towards the end, with comic relief Roscoe Ates using dynamite to blow his own clothes off, leaving him in long Johns and blackface. Yet notice, if you will, that the Indians, who don't do much of anything, are not portrayed as bloodthirsty savages. While there are plenty of westerns in the era which do so, it was not an invariable trope before BROKEN ARROW supposedly broke the mold. The racism towards Indians is not an assumption of the film makers here, but part of the story.
And yes, Lash does use his whip during the big fight at the end.
