The shadow imagery that is employed in these chapters comes to refer to the past suffering of Dr. Manette, but also how this suffering still casts a shadow on the future of Dr. Manette and, in particular, his son in law and daughter.
The first mention of shadows comes in Chapter 17, which records a conversation one night between Dr. Manette and Lucie after Darnay and Lucie´s engagement has been confirmed. The irony of this conversation is that Dr. Manette talks to Lucy about his suffering for the first time, showing that he has began to put it behind him. Talking of the inevitability of her engagement, Dr. Manette says:
"Or, if it had been no other, I should have been the cause, and then the dark part of my life would have cast its shadow beyond myself, and would have fallen on you."
Ironically of course, we later find out that the precise reason for Dr. Manette´s suffering in the Bastille is of course a shadow that does fall upon Lucie and her husband, because of Dr. Manette´s involvement with the Evremonde family. However, more immediately, the shadow comes to fall on Dr. Manette again when Darnay and Lucie leave for their honeymoon in the following chapter. We are told that after a private conference with Darnay, he emerges changed:
He was so deadly pale - which had not been the case when they went in together - that no vestige of colour was to be seen in his face. But, in the composure of his manner he was unaltered, except that to the shrewd glance of Mr. Lorry it disclosed some shadowy indication that the old air of avoidance and dread had lately passed over him, like a cold wind.
Dr. Manette´s "relapse" in this Chapter does not bode well for the future of his daughter´s marriage, and is used to foreshadow the imprisonment and potential threat against Charles Darnay that "shadows" him. Thus the shadow imagery is used to foreshadow future disaster and threats. Although Dr. Manette has tried to put his past suffering behind him, the reasons behind that suffering, and in particular the relationship and responsibility of the Evremonde family of that suffering, means that his experience of imprisonment continue to dog both him and his daughter and son-in-law.