The period from 1775 - the outbreak of the American Revolution - to 1789 - the storming of the Bastille - is the turbulent setting of this uncharacteristic Dickens novel. It is his only novel that lacks comic relief, is one of only two that are not set in nineteenth-century England and is also unusual in lacking a primary central character. London and Paris are the real protagonists in this tale, much as the cathedral was the 'hero' of Hugo's Notre Dame de Paris. Dickens was writing at a time of great turmoil in his personal life, having just separated from his wife, and no doubt the revolutionary theme was in tune with his mental state.
The result is a complex, involving plot with some of the best narrative writing to be found anywhere, and the recreation of revolutionary Paris is very convincing. The device of having two characters that look identical may seem hackneyed to modern readers, but it is here employed with greater plausibility than in Twain's Pudd'nhead Wilson or Collins's The Woman in White.
Dickens was inspired to write this story by reading Carlyle's newly published history of the French Revolution. Those events and their aftermath stood in relation to their time much as World Wars I and II do to ours, that is, fading from living memory into history, yet their legacy still very much with us. In many nineteenth-century novels, especially Russian and British works, you get a sense of unease among the aristocracy that the revolution will spread to their own back yard. In the case of Russia, of course, it eventually did.
I have often recommended A Tale of Two Cities as a good introduction to Dickens for younger readers. This is based on my own experiences, because it was a set book in my English Literature class when I was 15 and I remember thoroughly enjoying it. Yes, it is challenging, with its somewhat archaic language and its slow development, but you cannot progress to an enjoyment of great literature without being challenged.