Slim Mantell is a trusted top hand on the JA Ranch.
Slim is one of two men that rancher Amarillo Ames considers trustworthy and hardworking enough to be on the short list to lead his JA Ranch.
Slim signed on to a cattle drive with Ames to Dodge City. Just before the drive started, Slim was cooling his heels outside the ranch house on the JA when a tall, dark-haired stranger rode up. Slim finds it pretty hilarious when the stranger brushes off Rowdy Shane's tough guy act and pushes by him to enter the house.
The stranger, named Cheyenne Bodie, joins Slim, Rowdy and the other JA men on the drive. However, the drive soon experiences tragedy when Ames falls into the river and drowns. After he helped bury his boss, Slim questions Rowdy's assumption of command but he is literally overshadowed when Cheyenne challenges Rowdy for leadership. Slim watches in trepidation as he and Rowdy fight, and is obviously disappointed when the stranger triumphs. He tells Cheyenne that he will stay with the drive, but not out of fear.
Cheyenne proves to be a hard boss issuing hard orders and Slim is a calming voice among the hot-blooded men. In a moment when they are alone, he tells Bodie that his loyalties lie with his friends, not the boss of the drive. Internally though he suspects that Cheyenne has an ulterior motive for his rough behavior.
Later, Slim is riding with Cheyenne when Ruck Grover's rustlers start after the herd. Somehow, Bodie knew they were coming and starts a reverse stampede that scares the rustlers off. However, the loss of one of their own in the attack spooks the men. At camp, Rowdy, Slim and the others tell Cheyenne they quit and demand their pay. Cheyenne refuses and instead encourages them at gunpoint to stay with the drive and take bonuses. Not wanting to be shot, Slim and the men accept the deal. Slim asks Cheyenne if his reasons for being so harsh are motivated by money, Cheyenne won't give him a definite answer. Slim expresses surprise and disappointment at this since he thought there was more to Cheyenne's motivations than money.
Later, Susan Ames catches up with the drive and Slim offers his condolences to her on the death of her father. Slim watches as she tries to put Rowdy in charge and fire Cheyenne, but he refuses to accept her authority and no one is willing or able to take him out by force.
The drive makes it to Dodge City first and bonuses are awarded by the city fathers to Susan and Cheyenne. To celebrate the end of the trail, Slim and the other JA men spend the evening at the saloon. Cheyenne walks by and Slim tells some of the angry men to let their grudges go and accept Bodie at their table. When he reminds them all that they're late to accept their pay, Slim and the others hurry out.
At her hotel room, Susan passes out Slim's money and a secret: Cheyenne drove him and the others so hard in order to earn the money that will save the JA Ranch from foreclosure, and because he felt he owed the Ames family due to a tragic accident 18 months before. Rowdy tells the men that Cheyenne is headed to the Longhorn Bar to face an enemy from his past. They hurry down to the bar and see Cheyenne is being held at gunpoint at the entrance. While Rowdy moves to help Cheyenne, Slim and the others slip into the back of the bar. Cheyenne's enemies start shooting and Slim and the JA men fight back and win.
The next day, Slim joins Susan and his friends for the ride back to Texas. As they ride out of town, Cheyenne joins up with them for the ride home.