'Big River'

''Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot,'' reads a banner that opens ''Big River: The Adventures of Huckleberry Finn,'' at the American Repertory Theatre.

Despite this salty threat, ''Big River'' does have a motive, a moral, and a plot - and some nice tunes to boot. This is a warm and winning production of Mark Twain's tale about a boy who learns a thing or two about honor, friendship, and honesty while rafting down the Mississippi with a runaway slave.

William Hauptman, who wrote the book, cut out many of the story's racial slurs and sliced a clean path through the meandering plot. The three-hour show is still a mite pokey, but it's so good that you don't mind much.

The large cast plays an even larger Nicholas Nickleby-ish variety of characters - from pious folk to rapscallions. Ben Halley Jr. strikes just the right note of dignity as the long-suffering Jim, and Robert Joy is an appealingly unaffected and boyish Huck. Kudos go to John Bottoms as the chicken-limbed, melodramatic actor/swindler; Thomas Derrah as the wildly imaginative Tom Sawyer; and Karen MacDonald as a young widow.

The music is a generally toe-tapping blend of country-western, bluegrass, Dixieland, and gospel. Songwriter Roger Miller's (''King of the Road'') knack for clever patter is evident in some songs; others are undeveloped and repetitive. ''Free at Last'' is a dirge in chains; but a country-western lament is beautifully sung by Miss MacDonald, who's the only singer who really shines.

The spare but versatile minisets (by Heidi Landesman) slide in and out on tracks under a large sepia backdrop of the Mississippi.

This is a show that will appeal to adults as well as children: The morals are firm but unpreachy, and it deals with major social issues (slavery, parental vs. children's rights) without laying it on with a trowel. Throwing in a good rendition of ''Amazing Grace'' doesn't hurt either. (Through March 25.)