Through the plain, i.e. in the way from Mahanaim to Hebron; which for the most part was a plain country.
Related Commentaries of 2 Samuel 4:7
2 Samuel 4:7
7 For when they came into the house, he lay on his bed in his bedchamber, and they smote him, and slew him, and beheaded him, and took his head, and gat them away through the plain all night.