Who watches the Watchmen? A lot of people, if HBO has its way. Debuted in 1986 by the writer Alan Moore and the artist Dave Gibbons, the dark and gritty superhero comic “Watchmen” was enormously influential, helping the genre to grow up and get real. For HBO, Damon Lindelof’s new TV adaptation, which premieres on Sunday, is a chance to recreate the runaway success of its last adaptation of a major nerd-culture touchstone: a little show called “Game of Thrones.”

The reviews so far are positive, which is more than might be said for the 2009 film. But unlike the movie, this isn’t a straight adaptation: It takes place decades later, when many of the central characters from the original are dead or vanished. The series does, however, exist in the same alternate history, in which Vietnam is the 51st state, Robert Redford is president, and Richard Nixon is on Mount Rushmore.

Early episodes are cryptic; it helps to have a little background. Here’s a brief rundown of the most useful things to know before watching.