The next movie about scandal in Washington features an ending with a twist: the hero, a ruthless presidential assistant, admits the error of his ways and dedicates his life to God.

The saga of Charles Colson, the White House aide who plotted to smear the reputation of Daniel Ellsberg, will begin filming next month in Washington, starring Dean "Son of Flubber" Jones as Colson and former Iowa Senator Harold Hughes as himself. (Hughes and Colson became fast friends in a Washington prayer breakfast group Colson began attending during the height of the Watergate affair.) Hughes and David Shapiro, Colson's chief counsel, will be the movie's "heroes"

Based on Colson's book, Born Again , the movie was planned last December during a meeting in the Hollywood Hills home of Robert Munger, a Reagan supporter, ex-ad man and born-again Christian. Present at the meeting: Colson and John Dean.

"You have, according to a Gallup Poll, 70 million people who have had born again experiences," says Munger, who is producing the film. "And you have the President of the United States talking about the experience. Obviously it's something with broad appeal."

Adds Munger. "I think there's enough hate and violence and stirring up people to hate themselves. There's something to having a movie about a man's transformation from a rat to a Christian."

Footnote: If the movie is successful when it is released nex summer, Colson intends to donate his share of the profits to a prison fellowship organization he began in 1975 with Hughes.