THE INVOLVEMENT two years ago of the youthful publisher of Washington's Dossier magazine with the Moonies has always sounded like a movie plot. Now the real-life script has taken another twist that is promising high drama next month in the courtrooms of England.

The Rev. Sun Myung Moon's Unification Church is suing The London Daily Mail for libel over the reprint of an article first published in the U.S. in Esquire magazine about the alleged brainwashing experiences of 26-year-old David Adler.

Adler, son of Warren Adler -- novelist and owner of Dossier, a magazine for Washington society -- joined the Moonie movement during a vacation trip to California in January 1978. According to the Daily Mail article, the Moonie program persuaded him to turn his back temporarily on his family and his publishing business.

It was Warren Adler's painful and dramatic account of how he and his wife got their son back from the Moonies that The London Daily Mail picked up from Esquire.

A spokesman for the Unification Church in England, Michael Marshall, said in a long distance interview last week that the Moonies also have two similar libel suits in London currently against The Daily Times and The Daily Telegraph.

According to Marshall, the libel suit against the Daily Mail, which comes to trial in October, claims that it is not church policy to split up families, that the church doesn't brainwash people and that the church has been slandered by implications that it is a "sinister organization."

The Daily Mail suit is expected to be a milestone test case.

Other Fleet Street papers are watching closely because The Daily Mail decided to defend the case vigorously and not cave in as other papers have done when the Moonies went on the offensive.

The paper is spending a lot of money, and, in a dramatic turn of events, according to a London Daily Mail executive, has hired Intertel, that mysterious Washington-based organization that is the world's largest private intelligence network.

Pitting Intertel's agents against the Moonies promised a conflict that could end up a a big-budget Hollywood production.

Intertel is a subsidiary of Resorts International, which owns the large Atlantic City gambling casino.

Washington's "Godfather" of gambling, Joe Nesline, once described Intertel in a memo now in the possession of the Federal Bureau of Investigation as "a very cute outfit."

"Equipped with every known electronic device and will use it," Nesline cautioned. "When and if you have a meeting, you must protect yourself in that area."

Intertel's James Bondian image worldwide may also be riding on the outcome of the battle with the Moonies.