The story opens on an intensely warm June day in Belfast, Maine, where Donald John Ramsay, known as Don John, visits the elegant mansion of Captain Patterdale. Donald hopes to invite the captain and his son Ned on a trial trip of the newly finished yacht, the Sea Foam, which Donald's father has just built. During the visit, a suspicious visitor named Hasbrook settles a note, and a clerk named Laud Cavendish delivers a bill, before the household is startled by the hired man Michael collapsing from a sun-stroke. While helping, Captain Patterdale discovers that his green tin box, containing money, notes, and valuable papers, has been stolen from the library.
Following a sudden and tragic illness, Donald's father passes away, leaving the family grieving and facing financial uncertainty. Determined to fulfill his father's last wish to care for his mother and sister, Donald resolves to take over the boat-building business. He secures a commission from Mr. Rodman to build a sister ship to the Sea Foam, named the Maud, despite initial doubts about his youth. Working with skilled ship carpenter Kennedy, Donald successfully lays down the keel and begins construction, proving his capability as a master mechanic.
Meanwhile, Donald mysteriously comes into possession of a fine sailboat named the Juno and a considerable sum of money from the eccentric and wealthy Captain Shivernock. Although commanded to keep the gift a secret, Donald uses the proceeds to fund his expanding business. Tensions rise in the local yacht club as members prepare for the first grand regatta on Penobscot Bay, where the Sea Foam competes fiercely against the Skylark. After a series of thrilling races and rivalries, Donald demonstrates his superior sailing skill by handling the Sea Foam to a decisive victory over the Skylark.
As construction on the Maud races toward completion, dark clouds gather over Donald when stolen papers from Captain Patterdale's missing tin box are mysteriously discovered hidden in his shop. Accused of the theft, Donald faces severe suspicion, while Laud Cavendish attempts to shift blame and conceal his own dubious involvement with Captain Shivernock. The mystery deepens as local authorities investigate a violent robbery in a neighboring town and search for the missing valuables.
The climax culminates in the grand launch and trial of the newly completed yacht, the Maud, before the entire Yacht Club and townspeople. Amidst personal rivalries, particularly with the boastful and scheming Laud Cavendish, Donald prepares to command his vessel in the ultimate regatta. The narrative gently hints at the restoration of justice, the vindication of Donald's honor, and the bright future awaiting the young boat-builder and his supportive community.
