Naïve surveyor Alexandre LaBranche also needs help: kicked off the family plantation by his abusive father, he struggles until his uncle introduces him to Mirabeau Lamar, Texas' outgoing president. Lamar offers Alexandre a dubious commission to map the Rio Grande boundary, without mentioning that Texas has abandoned not only the river but the entire frontier to the Mexican army. Alexandre, desperate to prove his worth, immediately accepts.

He soon finds himself out of his depth and at the mercy of Texas' new president, a vindictive Sam Houston, who bullies Alexandre into becoming his unwilling spy. Houston saddles him with two bumbling horse thieves and sends him to prepare for spying on the Mexican army, led by Houston's enemy, Colonel Canales. Alexandre soon learns how inept a spy he is: he whiles away his time in San Antonio, socializing and sending obviously fake dispatches back to Houston, oblivious that the Mexican army has arrived on San Antonio's doorstep.
Canales, believing Alexandre is a surveyor for English banks that hold Mexican debt, demands that Alexandre continue to the frontier and report on how suitable the land is for a major invasion. Now trapped between Houston and Colonel Canales, Alexandre also is troubled by unsettling feelings he has developed for Noeme, a mysterious Black woman who seems to know more about Alexandre's business than he does. He proceeds to go into hiding from all of them. Months later, he discovers the Mexican army moving stealthily through the Hill Country to capture San Antonio for a second time.

Based on rich historical research and laced with exuberant, Texas-sized characters--including Sam Houston, Mirabeau Lamar, and Jack Hays, the first hero of the Texas Rangers-- Dangerous Latitudes is a quest across a war-torn frontier that becomes a race to save two hundred captured Texans marked for death.