In Area 51: The Sphinx ((Dell, $5.99), the fourth and latest entry in Robert Doherty's seven-book "Area 51" series, the action races from the Giza Plateau in Egypt to Easter Island and the Qian-Ling tombs of China.

Unfortunately, the globe-trotting fails to make the book much more than just the middle installment in a series, extending the plot of previous adventures but never managing to offer a satisfying conclusion in itself.

The premise of the "Area 51" series is familiar to conspiracy buffs.

Two competing groups of aliens have lived among humans for millennia, influencing a variety of historical events and spurring the construction of some of Earth's oldest artifacts and historical sites.

Lexina's terms are simple. The Americans must give her an alien key within 49 hours, or the satellite will rain high-radiation cobalt onto the United States, leaving it uninhabitable.

Unfortunately, the government doesn't have the key, forcing Turcotte and Duncan -- and other agents, historians and explorers -- to scour the globe for its location.

Area 51: The Sphinx relies on frenetic action and a style to match.

Doherty divides his sections into short, easy-to-read chunks, each of which leads off with a place and a time on the countdown-to-destruction-clock. We can almost hear the typewriter sound effects that accompany this much-used device in action movies.

What's more, "Doherty" is a pseudonym of Bob Mayer, who also writes military fiction. Mayer has served with infantry and special forces units, and clearly knows his ordnance.

The weapons are carefully and seemingly accurately described, and the military strategy against the aliens and their weapons seems plausible.

There's also a lingering sense of the Cold War. As a U.S. UFO research organization, Area 51 competes with its Russian counterpart Section Four, leaving soldiers and spies ambivalent about whether to cooperate with each other.

The Sphinx spends a lot of time extending the plots of the earlier novels, which provides a nice sense of continuity but takes away from the current book's narrative momentum.

This lack of drive shows up in various ways.

The material on the Pyramids, the Ark of the Covenant and Lexina's alien key -- which might just be the legendary Spear of Destiny -- is compelling, but it's written as exposition, more window-dressing than revelation.

This plot treatment also takes a toll on the novel's climax. We get to a big discovery and the book is over. That's it.

Doherty could have explored the ramifications of this discovery, but we are left with what feels like a "To Be Continued" caption.

As a result, instead of tuning in next week, we'll be waiting a year to find out how what we learned at the Sphinx affects the three books to come.