Eight oarsmen row Marguerite, Auguste, and Damienne through rough seas in the Gulf, passing several barren granite islands. The navigator Jean Alfonse repeatedly refuses to abandon them on lifeless rocks until they reach a larger island with grass. The men unload trunks and supplies, and Jean Alfonse offers parting words of hope, suggesting another vessel might find them and inviting them to La Rochelle if reunited. The boat rows away, leaving the three alone.

The castaways pray together, then scramble to save their belongings from the rising tide. They take stock of their possessions: trunks of clothes, linens, tools, weapons, food, seeds, instruments (Auguste's cittern, Marguerite's virginal), and small valuables. Damienne notes the soil is only peat, and the trees are stunted. Damienne prepares a signal fire, but Auguste warns her to wait and only signal French vessels, lest they attract enemies.

Auguste climbs a peak and reports the island is small, uninhabited, with no buildings or paths. While exploring, Marguerite falls and cuts her hands; Auguste washes them in fresh rainwater pools they discover. They follow a strange humming sound to a cove teeming with thousands of fearless white waterfowl. Auguste shoots one, and the trio flees with the kill, then roasts and eagerly eats their first fresh meat in weeks.

Searching for shelter, they find a waterlogged oak too hard to chop apart. Auguste detaches two large curving branches and, with Damienne's surprisingly capable help, drags them up to a granite ledge. Marguerite tries to assist but cannot lift the heavy stones; Damienne reveals she learned to dig as a child in her original home before service. They fashion a rough "cottage" with the branches and dried sail as a roof, arranging trunks as chairs around a hearth.

Damienne sleeps under the shelter on a peat bed, refusing to unpack sheets. Marguerite and Auguste keep watch together through the night, finding strange joy in being castaway together, beyond Roberval's reach. Auguste spreads his cloak on the leftover peat, and they consummate their love, with Auguste vowing none can part them now.