When does Falling Skies return?

The series stars Noah Wyle as Tom Mason, a former history professor who becomes the second-in-command of the 2nd Massachusetts Militia Regiment, a group of civilians and fighters fleeing Boston following an alien invasion. The aliens include mechanical attack drones called "mechs"; a species of six-legged beings known as "Skitters" that appear to control the mechs; and a mysterious species known as the Overlords, or "Espheni" The aliens' objectives are not explained until the fourth season.