When last seen, Diana Bishop – the time-walking witch and narrator of Deborah Harkness’ first novel, “A Discovery of Witches” – was hightailing it out of the 21st century with her 1,500-year-old vampire lover, Matthew Clairmont. Now, in the sequel, “Shadow of Night,” Diana and Matthew have touched down in 1590. They’re on the lam from one of those shadowy supernatural cabals beloved of genre novelists. Which shady organization doesn’t really matter: This novel proceeds at a snail’s pace, leaving a silvery narrative thread that’s almost invisible.

Fortunately, Harkness makes up for a lack of narrative thrust by weaving a tapestry of 16th-century European life that is as densely populated, colorful and occasionally puzzling as a painting by Hieronymus Bosch. A scholar of Elizabethan history, Harkness is an entertaining guide to a French chateau, teeming London and the Prague Jewish ghetto during the Tudor era. Numerous luminaries – Walter Raleigh, Queen Elizabeth I, Emperor Rudolf II – and lesser-known historical figures are all given the kind of walk-ons beloved of the old “Masterpiece Theatre” crowd. Like Diana and Matthew, some of these folks turn out to be supernatural creatures. Christopher Marlowe was a charismatic, sexually obsessive, substance-abusing demon? Of course!

What plot there is mostly revolves around the fugitive lovers’ search for Ashmole 782, an ancient, mysterious alchemical manuscript (is there any other kind?). Diana discovered the text back in our own century when she was a historian at Yale and realized it was missing three critical pages. Now she’s searching for the original, intact copy. In 1590, of course, it’s rare for a woman to know how to read and write, let alone interpret alchemical symbols. It’s even rarer for her to show her legs, and Harkness provides amusing and intriguing descriptions of 16th-century wardrobe functions and malfunctions.

“Shadow of Night” is overstuffed with secondary characters and plot elements that never quite earn out. Firedrakes, goddesses, waterwitches, plucky orphans and a golem all make an appearance. It’s not enough that Diana is on hiatus from modern times: In the past, she runs into her wizard-anthropologist father, too. A sojourn in Rudolf II’s Prague is a high point, but it comes almost three-quarters of the way through a novel that could easily have been half as long. Would that a strong editorial hand could be summoned as easily as a firedrake.

Still, “Shadow of Night” makes a satisfying beach read, with enough surprise cameos and fun facts to offset its longueurs. Thomas Harriot, legendary mathematician and astronomer, was a demon?

Who knew?