A HALF-TIME scoreline that promised so much turned into disappointment for Argyle as two goals, in the final 10 minutes, saw their Devon rivals leave Langsford Park with all three points.
The Pilgrims led 2-0 at the interval, having scored through Matt Ward and Tom Purrington, but a second-half comeback by Exeter turned the game completely on its head.
Exeter had plenty of possession in the first half and probably missed the best chance of the game, on 22 minutes, when Josh Key fired over a great low cross that Joe O'Loughlin somehow hit wide from close range.
Argyle made their visitors pay for that miss as great play down the right bye-line by the impressive Alex Fletcher set up Ward to fire home from 10 yards on 29 minutes.
It was 2-0 six minutes later as Cameron Sangster's right wing corner was headed down by Fletcher for Purrington to hook the ball home from close range for his first ever goal for the under-18s.
The second half began with O'Loughlin heading over the Argyle bar before the same player was denied by a fine near post save by Michael Cooper.
Argyle almost grabbed a third but Ward's effort was blocked by goalkeeper Alan Siviter's legs before Exeter pulled a goal back on 70 minutes. Elliott Crawford's tackle on Archie Colllins was deemed a foul, and skipper Max Smallcombe just beat Cooper from the penalty spot.
Max Childs then replaced Cooper in goal but within four minutes he was picking the ball out of his net as Key's fierce cross-shot flew across him and into the far corner of the net.
The game was in stoppage time as Key found space in the area to turn and fire inside Childs' near post for the Grecian's winning goal.
Argyle: Michael Cooper (Max Childs 76), Tom Purrington, Rio Garside, Aaron Taylor, Jordan Bentley, Elliott Crawford, Billy Craske, Matt Ward, Alex Fletcher, Cameron Sangster, Alex Battle (Mani Skett 78). Substitutes not used: Sam Morgan, Michael Peck, Jack Crocker.