AT their best, the "Star Trek" movies cleverly mix Hollywood-style escapism with sly comments on life in our own century.The finest example was "Star Trek IV: The Voyage Home," in which the starship Enterprise touched down on Earth for a first-hand encounter with the 1980s, providing more laughs and sheer humanity than any other entry in the series. Runners-up include "Star Trek V: The Final Frontier," which dealt with the need for spiritual enlightenment in a high-tech era, and "Star Trek III: The Search for Spock," which blended the usual pyrotechnics with a melancholy tone and a mellow visual style. "Star Trek VI: The Undiscovered Country," billed as the last of the series, is the first to be released since the death of Gene Roddenberry, who dreamed up the whole Trekkie idea some 25 years ago. Unfortunately, it's a letdown compared with the installments mentioned above. It starts with a promising angle, portraying the perennial conflict between the Federation and the Klingons as an allegory for real East-West relations. But the screenplay does little to capitalize on this. The result is an ordinary science-fiction adventure. The tale begins with a disaster striking the Klingon empire - not a random disaster, but a mishap recalling the Chernobyl incident that shook the Soviet Union several years ago. We then discover that the Klingons will soon be too broke and beleaguered to sustain the empire they've dominated for so long. To take advantage of this unexpected change, the Federation has decided to broker a lasting peace with its longtime enemy. In a surprise move, the person delegated to initiate this is Captain Kirk, whose hostility toward Klingons has festered ever since they caused the death of his son. And this raises some interesting questions. Can the most dedicated Klingon-hater in the Federation overcome his bigotry and be diplomatic, or at least civil and well-mannered, during a conference over the Enterprise dinner table? Or will his prejudice ruin the Federation's chance to end a long and damaging conflict? It's clear that "Star Trek VI" has borrowed its theme from recent changes in the real world, including the decline of the Soviet economy, the break-up of the Eastern bloc, and the end of the cold war. It was probably wise of the "Star Trek" screenwriters not to pursue such connections too literally - by introducing Gorbachev or Yeltsin characters, for instance, or making the plot echo real newspaper headlines. But they could have done something creative with their basic idea. Instead they only use it as a starting point, and then veer off in very different directions. The similarities between fact and fiction reappear at odd moments, as when two of our heroes (Kirk and Bones) are imprisoned on the Klingon equivalent of Siberia, a "penal asteroid" where it snows all the time and prisoners work in underground mines. Most of the show is just a string of typical Trekkie adventures, however, until a preachy finale telling us that peace is an "undiscovered country" waiting to be explored. On its own limited level, "Star Trek VI" is a well-made entertainment. It was directed by Nicholas Meyer, the auteur of "Star Trek II: The Wrath of Khan," and photographed by Hiro Nirita, with attention to human as well as technological values. The cast is comfortingly familiar, complete with William Shatner as the Enterprise commander, Leonard Nimoy as his dispassionate Vulcan colleague, and the usual lineup of supporting players. The movie they give us is hardly an undiscovered country, but it's pleasant to have one more visit there. And it's tantalizing to ponder the biggest question of all: Is this really the last of the series? Could be, but don't bet your phaser on it.