The 20-year-old joins the club after being released by Oxford United at the end of last season, where he gained plenty of experience through loan spells.
Plumley never made a senior appearance during his time with Oxford, but he was named on the bench seven times during his spell with the League One club.
The boyhood United fan also gained valuable experience during loan spells with lower league clubs Weston-super-Mare and Beaconsfield.
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Plumley is expected to occupy a back-up role in United&aposs youth sides over the coming season and has been registered as one of the eligible U21 players on the club&aposs Premier League squad list.