Tellson's Bank has one office in London and another in Paris. Now, the books and papers of the Paris branch are in peril and Mr. Lorry is traveling there to retrieve these books and papers, preventing them from falling into the hands of the revolutionaries (the Jacqueses). In addition, such a passage of these documents and papers is very dangerous at this time.
While Darnay is in the bank, he hears talk of a letter addressed to the Marquis St. Evremonde, his French name. This name, in agreement with Dr. Manette, is to be kept secret. Saying that he knows the man, Darnay takes the letter; he walks away and opens it. In this letter, Gabelle, who was like an overseer of the Evremonde estate, has remitted the imposts and collected no rent. Yet, he has been arrested and his house razed to the ground from fire. Gabelle is charged with treason against the people and for "acting for an emigrant [Darnay]." He begs the Marquis to return to Paris on his behalf.
After witnessing the sneers of the Monseigneur and the bruskness of Stryver, Darnay reflects
Upon those, had followed Gabelle's letter, the appeal of an innocent prisoner in danger of death, to his justice, honour, and good name.
Darnay feels compelled to return.
Mr. Lorry is going to Paris to check on business for Tellson's Bank. Despite the ongoing revolution in France (it's 1792), Mr. Lorry wants to check on the property of the bank's French customers, and he is bringing Jerry Cruncher with him to protect him. Communications between France and England have been disrupted, so Mr. Lorry needs to attend to business by going to Paris.
Tellson's then receives a letter addressed to "Monsieur heretofore the Marquis St. Evremonde." Without revealing that he is related to Evremonde, Charles Darnay tells Mr. Lorry he will deliver the letter to Evremonde. When he opens the letter, Darnay finds it is from Gabelle, who has been imprisoned in France. Tasked with carrying out tasks for the Evremonde estate, Gabelle has been charged with acting in a treasonous way against the people for the sake of helping an emigrant. Gabelle pleads with Evremonde to come to rescue him from prison and to explain that Gabelle was acting in accordance with Evremonde's commands.