As the days grow cooler and the nights longer, mysterious things begin to happen, most definitely at The Little Theatre of Alexandria anyway. Opening the fall season with Agatha Christie's "Witness for the Prosecution," the mood is appropriately intriguing.

Agatha Christie fans will leave this production feeling smug. The opening set is appropriately refined, complete with shelves of leather-bound books and brass chandeliers in a room of pine green walls. In short, just the kind of room you would imagine exists in the English countryside.

The plot twists and turns, carefully misleading the audience. A rich, elderly matron is murdered on the maid's night out. The suspect: a young married man struggling to pay his bills, who we learn befriended the victim in recent months. Yet there are others who are acting too nervous for their own good. Ultimately, the final scene sends each amateur sleuth home completely surprised.

The cast indulges with zeal in the very British dialogue. The characters and their mannerisms are perfectly executed, with special notice to Jay Mattlin in the role of Mr. Vole, the prime suspect. However, every now and then an actor loses the English accent, which results in distraction.

"Witness for the Prosecution" was originally a short story. Opening for the first time at the Royal Theatre Nottingham in 1953, the stage adaptation was not a big hit. When the production reached the Winter Garden Theatre in London later that year, however, it was a great success. There is nothing like a good whodunit to warm up a cool and windy autumn night.

"Witness for the Prosecution," The Little Theatre, 600 Wolfe St., Alexandria, through Oct. 1. For more information, call 683-0496.

Elsewhere:

Reston Theatre presents The Shenandoah Conservatory in concert at 8:30 p.m. tomorrow. Saturday,Euterpe chamber music. For more information, call 476-4500.