THE COPLT REPORTED THAT THE ACFT HAD BEEN IN LEVEL CRUISE FLT APRX 1 MIN WHEN THEY FELT A VIBRATION, FOLLOWED BY A LOUD BANG. THE AIRCREW SAW FIRE & SMOKE COMING FROM THE LEFT ENG, SO THEY SHUT DOWN THE ENG & FEATHERED THE PROPELLER. THE PLT ALSO ACTIVATED THE FIRE WALL CUTOFF SYS & THE FIRE EXTINGUISHER, WHICH HE SAID, EXTINGUISHED THE FIRE. THE AIRCREW DIVERTED BACK TO THE DEPARTURE AIRPORT, BUT WHEN THEY TRIED TO EXTEND THE GEAR, THE LEFT MAIN FAILED TO EXTEND & THE RIGHT MAIN ONLY PARTIALLY