"The Atavist" tells the story of Steve "Dev" Devlin, a man whose wartime experiences have left him restless and out of place in civilian life. He meets his war buddy Jimmy Kelliher in New York, who introduces him to the boxing scene. Despite having little formal training, Devlin's innate fighting spirit shines through, and he's given an opportunity to fight in a preliminary match against Battling Barron, a well-known slugger, after a chance meeting with a fight promoter, Stanton.

During the match, Devlin's natural prowess and savagery come to the fore, leading him to an unlikely victory that sets his boxing career in motion. Under the management of 'Lynx' Sloan, Devlin quickly ascends to the welterweight championship, demolishing contenders with his exceptional speed, strength, and 'knock-out kick'.

Despite his success, Devlin yearns for a challenge and sets his sights on the middleweight champion, Tommy Jansen. However, Jansen continuously avoids Devlin, fearing defeat. In an attempt to force Jansen's hand, Devlin takes on fights with middleweights, including Johnny Burrone, whom he defeats with difficulty, proving his strength extends beyond his weight class.

Devlin's relentless pursuit of a fight with Jansen leads to him being manipulated into a match with the light heavyweight champion, Ghost Donovan. Devlin harbors an irrational hatred towards Donovan, whom he views as a symbol of everything he detests—a fighter who seems too polished and engaged with society, contrary to Devlin's own rough and singular focus on combat.

The narrative explores the contrast between Devlin's atavistic nature—a throwback to the primitive, pure fighters of old—and the modern world's multifaceted individuals like Donovan. The story, unfinished, leaves readers anticipating the clash between these two champions, both within the ring and in the ideological battle of what it means to be a fighter.