In The Mysterious Island, five people crash-land their hot air balloon onto an island while escaping the Confederate stronghold of Richmond during the American Civil War. They are the engineer Captain Cyrus Harding, the reporter Gideon Spillet, Harding's black servant Ned, the sailor Jack Pencroft, and fifteen-year-old orphan Herbert Brown. Pages 14 to 20 contain highly detailed descriptions of the characters, including this description of Captain Cyrus Harding.

One of the most distinguished was Captain Cyrus Harding. He was a native of, Massachusetts, a first class engineer, to whom the government had confided, during the war . . . A true Northerner, thin, bony, lean about forty-fiftv years of age; his close-cut hair and his beard, of which he only kept a thick mustache, were getting gray. He had one of those finely-developed heads which appear made to be struck on a medal, piercing eyes, a serious mouth, the physiognomy of a clever man of the military school.

Under the leadership of Harding, the men colonize the island, which they name Lincoln Island, and begin to make it their home.

In chapter sixteen, they are joined by a sixth character called Ayrton. Initially thought of as a wild man, he confesses he is in fact the sole survivor of a ship called the Britannia, on which he lead a mutiny, leaving the captain stranded on an island. Harding shows what a good man he is by forgiving him of his crimes.

You have been a great criminal, but Heaven must certainly think that you have expiated your crimes! Ayrton you are forgiven! And now you will be our companion?

The final major character in the book is Captain Nemo, the main character from Verne's novel Twenty Thousand Leagues Under the Sea, who first appears in chapter sixteen. Nemo saves the colonists from pirates and gives them some of the valuables he earned as a nobleman in India. Before dying, he tells them,

In the hands of such men as yourself and your comrades, Captain Harding, money will never be a source of danger.