It’s a steamy summer afternoon in Washington and the National Mall is packed _ and not just with the usual protesters, tourists and schoolkids. It’s been overrun by bulbasaurs, charmanders and rattatas, not to mention the smartphone-wielding hunters bent on tracking them down. If that last sentence sounds like gibberish, you’re immune to the hottest app in years. ” Pokemon Go ” (Nintendo/Niantic, free, for iOS and Android) has not just swept the nation, it’s taking over the world. That says a lot for the lingering affection many have for the adorable “pocket monsters” that first came to life 20 years ago _ because the game itself is kind of a mess. The premise is irresistible: Pokemon burst into the real world. While you walk around your neighborhood, the little guys pop up on a map on your phone. When you tap a creature, the app switches to your camera; hold up your phone and you can see the creature floating in front of whatever landmark you’re actually standing in front of. Then you have to catch the critter by swiping on your phone to fling a “PokeBall” at it. It’s a weird sensation, seeing these familiar characters suddenly inhabiting familiar spaces. (The Secret Service really needs to do something about all the zubats flying around the White House.) But that’s about it: You can’t really do much with a Pokemon once you catch it. Essentially, “Pokemon Go” boils down to a worldwide scavenger hunt. That’s not a bad thing in and of itself. Anything that forces a couch-bound gamer like me to get outside and exercise is a positive. And the game’s maps are dotted with “PokeStops,” where you can not only grab Pokeballs and snacks for your creatures, but you can also learn a little about local landmarks. Beyond the “collect-’em-all” mechanic, you can also join teams of like-minded collectors. Then you and your teammates can try to take over “gyms” (there’s a big one at the Washington Monument) by populating them with as many of your Pokemon as you can round up. Fans of Nintendo’s earlier video games will miss their strategic battles; you can fight other Pokemon at gyms, but the technique is far more simplistic. And if you grew up with the collectible card game _ well, “Pokemon Go” gets the “collectible” part right, but the head-to-head competition is absent. Still, the real-meets-virtual “augmented reality” of ” Pokemon Go ” is so nifty, I’m inclined to be generous _ if the app itself wasn’t so glitchy. Most of its issues stem from the understandable fact that developer Niantic had no idea “Pokemon Go” would be so popular. The game’s servers are so jammed that I’ve been able to log on maybe once out of every 10 attempts, and most of those sessions ended with the game crashing. Those problems can be fixed. For now, though, I don’t find the core of “Pokemon Go” _ walking and collecting _ interesting enough to endure them. Nintendo and Niantic are onto something special here, but its staying power will depend on whether they add some more compelling gameplay. One-and-a-half stars out of four.