THE EMPLOYEE WAS REFUELING TRACTOR. UPON COMPLETING FUELING, THE EMPLOYEE WAS RETURNING THE FUELING HOSE TO THE INSIDE OF THE BERMED FUEL STOR- AGE TANK AREA. THE EMPLOYEE STEPPED OVER THE BERM WITH HIS RIGHT FOOT CONTACTING EXPOSED PLASTIC SHEETING. AS EMPLOYEE'S WEIGHT SHIFTED THE RIGHT FOOT SLIPPED ON THE PLASTIC CATCHING ON UNEVEN SURFACE CAUSING ANKLE TO TURN INWARD.