After having the better of the first half with Le Fondre hitting the post and centre half Michael Raynes seeing his header cleared off the line, the Millers fell behind just after the break.
New signing and former Miller Will Atkinson netted on his Argyle debut as he cut in from the left and beat Conrad Logan with a low right foot shot.
But Evans and Le Fondre stole the show thereafter, Evans replying just three minutes later as he drilled in left footed after racing clear of two defenders from 20 yards.
Alfie then crossed from the right to set up Evans' second as he powered a header into the corner, before getting in on the act himself.
Lewis Grabban had a goal disallowed for offside and the tireless Jason Taylor had a goalbound shot blocked before Alf slotted in a third.
Taylor then capitalised on a defensive error to slide the ball to Le Fondre who clipped in the fourth as the Millers wrapped up their first win in 17 attempts at the Devon club to make it a 100% start to the season in League Two.