A Wandering Mind: Why I'm not following Tale of Two Cities

There must be something wrong with me or else I'm very distracted; Dickens's A Tale of Two Cities is supposed to be pretty easy reading, right? High-school kids have read it since 1849 or so. But for some reason I'm having a lot of trouble holding the plot elements and the characters in my mind. I know that the opening is meant to be mysterious, as Dickens introduces a number of characters and intentionally doesn't tell us much or anything about who they are or what their background or connection is or for that matter why we start on the night coach to Dover, presumably en route to Paris. We are to piece these things together and make the connections as the plot coheres in subsequent chapters - but I'm still confused (had the same problem a few years ago when I tried to read Our Mutual Friend, though that novel was far more dense - Tale at least is easy to read chapter by chapter, with a lot of excellent dialogue and the characters sketched in broad strokes). I'm going to have to look at a plot synopsis sometime later, but here's what I recall, and don't: Lorry, a banker in a private firm (Tellson's?) is en route to Paris via Dover when a messenger overtakes his coach and gives him the message: Returned from the Dead. Shortly, Lorry arrives in Paris where he finds man in an old apartment in a state of post-traumatic shock; he's been freed from prison after 18 (?) years, and forgets who he is and thinks he's a cobbler; the man in Mannette, and his daughter, Lucie, takes charge of him: presumably he's the one back from the dad, but I don't know what the message was about, what he had done to be imprisoned, any connection between Lorry and the Mannettes. Next stop: 5 years later (1780); a man Darney (?) is on trial as a French spy - Lorry is in the gallery, the old Doctor Mannette, still tended by daughter, is a key witness, Darney is acquitted (still don't see all the connections). Lorry afterwards visits the Mannettes; he's obviously in love with Lucie, but he's very formal and shy. We also follow the lawyer who won the case, who's a sleazy and aggressive character (Stryver) and his sidekick, a mean alcoholic (Carton) who played some role, not sure what, in the trial. Secondarily, we have met a guy who's a messenger for Lorry's firm who was called to the trial to give Lorry a message; not sure why - can't remember his name but it's something like Cruncher. How do all these strands tie together? I have no idea at this point. I'll keep going - out of faith in Dickens and out of shame at my own wondering attention.