An Unruly Little Animal unfolds on the most stressful of February days for young worrier Darby Tamm. In the morning, the fifth grader must deliver a pair-presentation with classmate Jennie Phelps-Christianson, and in the afternoon, he must endure a class visit and career-talk on mechanical engineering from his recently estranged father. Additionally, other stressors complicate his day: a) Darby's original fifth grade teacher Mr. Henderson, who had a car-accident-of-unknown-origins back in November, has died that night before, b) the class's substitute teacher Monsieur Substitute has been having an affair with his classmate Jennie's mother Vice Principal Phelps-Christianson, a secret Darby shares with former-friends-turned-dangers Alexander and Them and a secret he must hide from Jennie, and c) throughout the day, Alexander and Them threaten to beat the snot-stuffing-piss-puke out of him and threaten to expose the fact that Darby has stolen from his father's collection of vintage pornography magazines. The complications confounding Darby intensify throughout his day, culminate in a classroom scuffle, and result in a better understanding of his mom, mechanical engineering, sex, and violence.

Comic, circadian, and literary, the novel's structure alternates between chapter and composition sections: single-day chapters that trace the increasing complications of Darby's day are interspersed with composition sections that reflect and riff on Darby's worried state of mind. Taken altogether, An Unruly Little Animal offers readers a poignant and humorous coming-of-age story.