In Chapter XXIV, entitled "Drawn to the Loadstone Rock," Charles Dickens alludes to The Rime of the Ancient Mariner by Samuel Taylor Coleridge and the Loadstone Rock, a mythical rock which somehow magnitizes ships and brings sailors to their dooms. In this same way, Charles Darnay heads to his doom by returning to Paris in response to the desperate letter of Gabelle.
The "loadstone" that pulls Darnay back to Paris is the "sins of his father" and uncle who tortured and killed peasants on their properties. When he was a little boy, his mother came to Dr. Manette and hoped to find the sister of the murdered boy alive, but it was too late. Later, when Charles talks to his uncle insisting that the Evremonde family has acted wrongly, he tells the Marquis that he must
...obey the last look of my dear mother's eyes, which implored me to have mercy and to redress....
This "loadstone" draws Darnay back to Gabelle and the peasants because of the obligation he feels about his mother's last wishes. Also, he feels obligated to vindicate Gabelle who has collected no taxes from the peasants per the directives of Darnay, thus, he has committed no offenses and is innocent of the charges against him, "treason against the majesty of the people."