Two young United States naval officers aboard a gunboat patrol the Yangtse River during rising anti-foreign violence, answering urgent calls to protect missionaries and consular outposts. Local diplomacy breaks down amid treachery and factionalism, and several foreigners are seized and imprisoned. The officers and their allies devise a daring escape and undertake risky river operations to bypass enemy batteries. Reinforcements join the effort, a coordinated assault rescues the mission, and hostile forts ultimately surrender, after which the participants sort out the events and their immediate consequences.