Watch Old Oklahoma Plains for Free: How to Stream the Movie Online at 123Movies. . One-time cavalry officer Rex Allen, between jobs as a star rodeo rider, is asked by his former commanding officer, Colonel Bigelow, to help settle a dispute between the army and local ranchers. The cavalry has commandeered a large parcel of land needed to test their newly-designed tank and prominent rancher Jenson has encited the locals to rebel at this intrusion. It is up to Rex and his sidekick, Slim, to thwart Jenson and convince the residents that these army tests are essential.