FOREVER PLAID. A musical by Stuart Ross . Director TBD. September 8-September 23. This smash-hit, record-breaking musical is a joyous celebration of four-part harmony groups at their most entertaining! Singing in the closest of harmonies, squabbling boyishly over the smallest intonations, the Plaids – Sparky, Smudge, Jinx & Frankie -- are endearing, hilarious singers who bring you a series of well-loved songs and right-on patter that will keep you laughing and humming along to some of the greatest nostalgic pop hits of the ‘50s & ‘60s. PREVIEW: TBD. Friday/Saturday shows $30. Sunday matinee $25.