When new comic books arrive in stores on April 17, retailers who ordered the latest issue of a Buffy comic will get a bonus: free copies of Issue No. 0 of Angel, a new series.

“We wanted to do something special with the launch of Angel,” Arune Singh, the vice president of marketing for Boom! Studios, the publisher, said in an interview. The notion, he noted, became, “How do we do a Beyoncé album drop, but for comic book stores?”

Keeping Angel under wraps was no easy feat. Retailers and fans hear about new comics months in advance and sometimes even earlier. Boom! worked with Diamond Comic Distributors, the hub for retailers, on the top-secret project — playfully code-named Tooth Fairy, after the film starring Dwayne Johnson, a.k.a. the Rock, as a hockey player. (Angel is a hockey fan.)

Angel, a vampire with remorse, will expand the universe of Buffy the Vampire Slayer, which was rebooted in January. Like the new Buffy series, Angel will be set in contemporary times, not in 1999 when the character’s television series — a spinoff of the 1997 “Buffy” TV show — first aired.