A Tale of Two Cities is a historical novel by Charles Dickens, set at the time of the French Revolution.
In the Third Book, "The Track of a Storm," Charles Darnay returns to France at the behest of a former servant. Darnay is then arrested and sentenced to death, as he is a French aristocrat who fled during the Revolution. Dr. Manette and his daughter Lucie, Darnay's wife, travel to Paris to help him. It is odd that they would be allowed back into Paris, as they too fled during the Revolution; one possible answer is that as simple citizens, rather than aristocracy, they are not on the radar to be captured and symbolically punished in service to the revolution. This interpretation is supported by the use of Dr. Manette's testimony in Darnay's trial; if the French Government had a problem with his being in the country, his testimony would certainly have been invalid.
A more accepted interpretation is that the public will at the time was more powerful than Government intervention. As Darnay is an aristocrat, public will deems him to be sacrificed. Dr. Manette, however, was a political prisoner, and as the Revolutionaries have won the day, political prisoners cannot be condemned in the public eye.
"I have been a Bastille prisoner. There is no patriot in Paris--in Paris? In France--who, knowing me to have been a prisoner in the Bastille, would touch me... My old pain has given me a power that has brought us through the barrier."
(Dickens, A Tale of Two Cities, eNotes eText)
Here, Dr. Manette allows that his (apparently widely-known) experiences have given him a sort of political immunity, afforded by the public will and preventing legal action because of the drastic consequences of public opinion. Indeed, when Manette's name is mentioned in court, the audience erupts in a cheer, contrasting their reaction to Darnay's real name, St. Evrémonde. Therefore, while the ruling authority might be upset at Dr. Manette's presence, they cannot act against him.