The novel narrates the life of John ('Jack') Easy, from his birth up until his marriage as a young adult. Jack begins life as a spoilt only child, whose father, a gentleman and landowner, has inculcated in him the principles of equality and the rights of man, as well as a propensity to argue. This leads to a series of comic episodes in which Easy poaches fish and scrumps apples from his neighbours and attempts to justify himself by engaging in political argument with them. At the age of fourteen, Easy joins a privateering vessel as a midshipman. His perceivedly eccentric beliefs endear him to most of his fellow crew members, including Mesty, a former slave and African prince, who shares Easy's beliefs. The larger part of the novel follows his various adventures as he plays pranks on tyrannical officers and goes absent without leave, yet still manages to avoid punishment due to the indulgence of his captain. Towards the end of the novel, Easy comes to renounce his father's beliefs and, ultimately, buys his own privateering vessel and marries an Italian woman.
Midshipman Easy is in part a satire on the political doctrine of the rights of man yet at the same time it is an irreverent farce which seems to portray the rebellious anti-authoritarian characters more sympathetically than most of those in positions of power.

Mesty (full name Mephistopheles) speaks a language that is a hybrid of creole, Irish English and American English: 'he had learned English with the strong brogue and peculiar phraseology of the sister kingdom, dashed with a little Yankeeism' (vol. 1, p.128). Mesty has numerous speaking parts throughout the novel including an extended narrative (vol. 2 p.156-64). However, when Mesty narrates the story of his life, the narrator interjects and comments that he will represent it in 'good English' as to do so in Mesty's 'mixed jargon [...] will be tedious to the reader' (vol. 1 p.276). In the main, Mesty's dialect is creole, with consistent th-stopping and copula deletion, though it also contains some Irish English discourse markers (e.g. 'Murder Irish') and respellings (e.g. 'pay' for 'pea').
While Mesty is portrayed positively if, at times, stereotypically, the other main dialect-speaking character, Mr. Easthupp, a former cockney criminal who has now become the steward's purser and who affects the status of a gentleman, is represented very negatively. Much of the dialectal content of his speech consists of h-insertion and v/w transposition.
The novel also includes some dialect representation in the speech of two Hampshire gamekeepers (vol. 1 60-66) and a farmer (vol. 1 p.80-81) from the same region.

Unexpectedly nondialectal characters: All of Easy's fellow crew members, with the exception of Mesty and Easthupp, speak in Standard English.