The plot of "The Cassandra Crossing," now at area theaters, is at once so cluttered and ishevelled that it seems a wonder anyone in authority had the gail to brazen the mess out Perhaps the producers, Sir Lew Grade and Carlo Ponti, and the director, George Pan Cosmatos, were inspired by the lamentable example of their villain. He's a peculiarly deranged American security officer who responds to a possible viral epidemic aboard a Continental express by ordering the train toward certain destruction over an abandoned railroad bridge in Poland known, omimusly, as "the Cassandra crossing."

This conspicous overcompensator is played with terse soleminity by Burt Lancester, doing overtime this week as the chief troublemaker in inept suspense thrillers with excrudating polemical pretensions. The stimulataneous openings of "Twlight's Last Gleaming" may be linked to the arrival of plague ships at the same dock on the same day.

"Cassandra" resorts to even more hysterical and expedient subterfuges than "Gleaming" in order to mount rhetorical attacks on American policy-makers or cover-uppers from the transparent camouflage of an all-star cast and a conventional melodramatic premise. A trio terrorists enters the Geneva headquarters of "The International Health Organization" and attempts to plant a bomb. They fail, but one of them escapes after being exposed to an exotic, deadly germ in a secret laboratory. When he stows away on a passenger train, the train itself becomes a disease carrier and the innocent passengers potential victims.

At the outset one expects a reasonably straightforward thriller about the measures that might acturlly be taken to quarantine the train and protect against an epidemic. However, "Cassandra" is nothing as nothing as modest and intriguing as "Panic in the Streets" transferred to a train. The ostensible peril is revealed to be a red herring, the largest among a veritable plague of red herrings. The real menace is not the germ but officious, devious Lancaster, who somehow manages to manipulate things so that the train becomes a traveling concentration camp destined for oblivion.

Cosmatos and writer Robert Kats had collaborated earlier on "Massacre in Rome," a heavyhanded movie versions of Katz's book about a Gestapoatrocity. In some hideous, far-fetched way they have ended up superimposing the same story on their second film. Richard Burton's overzealous German has become Lancester's overzealous American, while Marcello Mastrioanni's ineffectual priest has become Ingrid Thulin's ineffectual doctor.

Cosmatos and Katz may be haunted by a particular nightmare fantasy, but they're not skillful enough to set up the nightmare plausibly. Cosmatos is an absentminded, huffing-puffing director who seems ot keep hoping we'll overlook his frazzled continuity, which suggests an old serial slapped together in such a way that the cliffhanging bits are never resolved. The plot mechanisms are less than scintillating. To cite one of the more imaginative examples, we're reminded that the contaminated terrorist could spread disease by seeing him drink form a doggie's water dish, lift a little girl, fondle an infant and cough in the rice bowl while passing through the kitchen.

The casting features such clever fake-outs as O.J. Simpson posing as a snide priest and Martin Sheen trying to find a handle on a gigolo who's really a dope smuggler who shows his essential cowardice only to turn right around and show his essential bravery. It's some comfort to report that Sophia Loren and Ava Gardner manage to sustain a semblance of glamor amid the wholesale casualties and confusion and despite the imbecilic dialogue. Cosmatos and Katz certainly leave one guessing.

The $64 Question: Does Lancaster also have a plan to eliminate all the friends and relatives of all the people he sends barrelling over the Cassandra crossing?