In a dramatic race, Plater was fortunate to avoid a clash with a slower rider with a minute to run.
Australian great Troy Corser was first away but Ballymoney man Dunlop soon hit the front on the three-cylinder MV Agusta, storming into the lead.
As a train of six riders battled it out for second place behind, Dunlop stopped with 11 minutes of the 25-minute race to go and handed the machine over to Plater.
The 2009 Senior TT winner, now retired from racing at the top level, brought the legendary Italian machine home to win by seven seconds.
Peter Bardell and James Haydon took second, just ahead of Michael Russell and Michael Rutter.
The second leg of the race at Goodwood will take place on Sunday.