In Chapter X of Book the Second of A Tale of Two Cities, Dr. Manette and Charles Darnay each make a promise to the others.
Dr. Manette's promise
A year after beginning his residence in England, Charles Darnay has fallen deeply in love with Lucie Manette. Respectfully, then, he approaches Dr. Manette to ask permission to marry her; Darnay prefaces his request with his acknowledgement of the close bond between the father and his daughter. Further, he declares that if his marrying Lucie were to "put any separation between her and you," he would not consider making his request as it would be base of him to do so. Then, Darnay asks a promise of Dr. Manette, who tells Darnay that he does not know how Lucie feels about him. He requests that if Lucie profess love for him that the father not speak ill of him. Dr. Manette promises,
I give the promise,” said the Doctor, “without any condition. I believe your object to be, purely and truthfully, as you have stated it. I believe your intention is to perpetuate, and not to weaken, the ties between me and my other and far dearer self. If she should ever tell me that you are essential to her perfect happiness, I will give her to you.
Manette further promises that if there be anything whatsoever against the man she loves, "they should all be obliterated for her sake."
Charles Darnay's promise
With his last statement, Dr. Manette hints at his knowledge of Darnay's background. In response, Darnay begins to tell him why he has changed his name, but Dr. Manette halts the young man's testimony. "You shall tell me on your marriage morning. Do you promise?" Darnay answers with one word, "Willingly."
So gravely has this converstion affected Dr. Manette that upon her return home, Lucie finds her father hammering at his shoemaking in his bedroom. Tapping at his door, she softly calls to him; when he comes, they walk together for a long time. Finally, Dr. Manette retires and sleeps well.