On March 29, 2004, the people of London were hit by a virus known as Shaun of the Dead. It spread quickly, reaching all of the U.K. on April 9th and the U.S. on September 24th. It was just a low-budget zombie movie from the creators of a British sitcom named Spaced — primarily, actor Simon Pegg and director Edgar Wright — but it became a massive success on both sides of the Atlantic. Now, two decades later, Shaun of the Dead is a genuine zombie classic, mentioned in the same breath as the George A. Romero films it was sending up.

But you can’t have a zombie movie without the zombies — especially a zombie movie like Shaun of the Dead, where the zombies aren’t only half the battle, but half the fun, too. And so, because Shaun of the Dead will be returning to theaters this weekend to mark...