Spring of 1856. Veteran of the Crimean War and the Nicaraguan Civil War, Samuel Kingston's guilty over his father's death withdraws from his loved ones, including his pregnant wife. Disagreement over baby's baptism exposes the religious rift between them. Sofia's fifteen-year-old brother, Filipe, arrives from Nicaragua, and Samuel shuns his news of the Filibuster War that killed his father; he wants nothing more to do with Nicaragua.
When his enemies, the Earls of Lucan and Sligo, freeze Samuel's assets, he plans to use the Baltimore documents incriminating them in Filibuster, William Walker's slavery plot. But Walker's agent, Colonel Fry, attacks the Kingston manor in Clonakilty and steals the documents. To save the family estate, Samuel travels with Padraig and Filipe, to New York, to accept Cornelius Vanderbilt's contract to support for the Central America countries fighting Walker's filibusters, but Vanderbilt rejects him.
In New York, learning the tycoon is in a race against Walker's allies to get arms for the Nicaraguan War, Samuel steals rifles destined for Walker, but Colonel Fry robs them back before he can sell them to Vanderbilt.
News that Sofia and baby have disappeared in Nicaragua, forces Samuel to take the only route open to pursue her. He smuggles weapons to Costa Rica; arms Vanderbilt is secretly supplying to President Mora's army. When they arrive in Costa Rica, the Costa Rican army imprisons Samuel, Padraig, and Filipe. A spy accused them of inciting rebellion against Mora's Costa Rican government.
To save their lives, Samuel convinces President Mora he has a plan to seize control of the San Juan River, Walker's lifeline to East Coast recruits. Rounding up the score of his Nicaraguan veterans exiled in Costa Rica, Samuel leads Colonel Brailier's Expeditionary Force through the jungle toward the frontier.
In a thrilling advance through the jungles, Samuel survives betrayal, deadly ambush, and imprisonment to seize the river forts and rescue Sofia. He forces Fry to return the Baltimore papers. Controlling the river, the Costa Ricans turn back hundreds of volunteers pouring down from America to join Walker, allowing the alliance of Central American countries defeat Walker. The US Navy transports the defeated filibusters back to America where Walker will face trial for breaching the Neutrality Act.
Samuel returns to Ireland and frees his fortune. Having learned religious bigotry is wrong, Samuel encourages Sofia to baptize John in her own faith.