He is Lucie's father, a brilliant physician, and spent eighteen years "in secret" as a prisoner in the Bastille prior to the French Revolution.
He is imprisoned because in the course of his medical practice he learns of abusive actions by two members of the aristocratic Evrémonde family.
While realizing the power at court of nobles such as the Evrémondes, Manette reports them to a minister of the royal government.
He is briefly given shelter in Paris by his former servant Ernest Defarge (who will subsequently be a leader of the storming of the Bastille) and is then reunited with his daughter Lucie.
Darnay is condemned for his uncle's sins, but Sydney Carton (out of love for Lucie Manette), disguises himself as Charles and takes his place in the guillotine and dies for him.