Preface and scope
Presented as a republication (originally “Rose Budd”), the piece is framed by its author’s expectation of protracted war with Mexico and a distrust of civilian interference in naval operations. It announces a moral through‑line: the commonplace duplicity of men—especially loud patriots—and traces a species of domestic treason enacted at sea. The narrative interweaves technical seamanship, salvage engineering, and legal/naval procedure with intimate moral testing.

Principal characters
Capt. Stephen Spike: rough, practised, avaricious master of the half‑rigged brig Molly/Swash, whose duplicity drives the plot.  
Harry Mulford: handsome, honourable first mate who deserts the brig on moral grounds, becomes pilot to the U.S. sloop Poughkeepsie, and Rose’s suitor and eventual husband.  
Rose Budd (and Mrs. Budd): ingenuous passenger whose perceptiveness grows; Rose is Mulford’s heroine and bride.  
Jack Tier / Mary Swash: long‑time ship’s hand who has lived disguised as a man; ultimately revealed as Spike’s abandoned lawful wife and beneficiary of salvage proceeds.  
Don Juan Montefalderon: well‑bred Mexican merchant, moderate and patriotic.  
The Poughkeepsie’s officers (Capt. Mull, Wallace, Rev. Hollins): embody organized naval authority, discipline, and moral adjudication.

Plot arc and key incidents
Departing New York ostensibly with a cargo of flour for Key West, the Molly Swash eludes revenue cutters and a sloop‑of‑war by seamanship and disguise. Mulford, suspecting illicit traffic—flour concealing gunpowder intended for Mexico—abandons Spike and later joins the Poughkeepsie. Off the Dry Tortugas the brig transfers part of her cargo to a Baltimore schooner; an accidental burst reveals gunpowder among the flour. A skirmish follows: Paixhans shells from the man‑of‑war detonate deposited powder, destroying much cargo and terrifying the women; Spike orders powder overboard to avert catastrophe.

A capsized, waterlogged Mexican schooner and subsequent salvage operations furnish the technical centerpiece: raising spars with slings and shores, salvaging a bag of doubloons, and the use of small‑boat work through shoals and breakers. A squall and a near‑complete sinking leave Rose, Mrs. Budd, Biddy, Jack and Mulford stranded on wreck and islet. Mulford’s nocturnal swim, improvised refloating of a boat, and rescue under shark‑threat exemplify resourceful seamanship. Jack, observing from aloft, aids by signaling; Don Juan conspires with Jack to manage appearances aboard Spike’s brig.

Climax and resolution
The Poughkeepsie’s arrival and Mulford’s testimony curb Spike’s schemes. In a desperate flight Spike broaches and grounds; his yawl’s crew cast several overboard to lighten the boat. The man‑of‑war’s cutter rescues survivors; Spike is mortally wounded and dies in hospital at Key West. Legal adjudication follows: recovered doubloons are distributed under prize procedures; Jack/Mary transitions back toward a conventional role and secures modest independence. Mulford is rewarded with rank and marries Rose at the lighthouse, embodying social redemption.

Themes and technical emphasis
The novella contrasts practical skill and moral choice: seamanship and salvage technique are portrayed as instruments of both livelihood and ethical testing. It condemns avarice and duplicity while privileging naval professionalism, humane rectitude, and the restorative power of lawful authority.