“Are we really going to Chernobyl?” That’s John McClane (Bruce Willis) asking his son, Jack (Jai Courtney), about the next stop on their mission to save the world. The answer is yes: They are going to Chernobyl and the “Die Hard” franchise is heading into radioactive meltdown.

It took 25 years, but with the fifth and latest entry, “A Good Day to Die Hard,” the series has finally devolved into joyless sludge. The 1988 original remains the very model of a modern action movie: smart, logical, loaded with firepower but rooted in the stressed vulnerability of Willis’s McClane, a little-guy cop against sneering Euro-terrorists. “Die Hard 2” (1990) is almost as good, and if 1995’s “Die Hard: With a Vengeance” is sloppy and spread out, at least it’s fun. With 2007’s “Live Free or Die Hard,” the series was back on firmer footing but also tending toward the ridiculous. That one lost me (and more than a few others) when the hero jumps onto the wing of an F-15 in mid-air.