Cardiff: Plunkett picked up four wickets, after Root, Buttler and Hales half-centuries, help England to 87-run win against New Zealand in a Group A clash of the ongoing Champions Trophy at the Sophia Gardens here on Tuesday.
England were first team seal semi-finals spot.
Jake Ball is Man of the Match for a decisive new-ball spell, and final figures of 2 for 31.
Victory meant England had won both their opening Group A games following their eight-wicket defeat of Bangladesh at the Oval last week.
New Zealand, set 311 for victory, were bowled out for 223 with 39 balls left in their innings.
Fast bowler Mark Wood took the key wicket of Blackcaps skipper Kane Williamson to dismiss the star batsman for 87.
Earlier, half-centuries from Alex Hales, Joe Root and Jos Buttler helped England to a respectable 310 all out in 49.3 overs
After being asked to bat first by New Zealand skipper Kane Williamson, England rode on an 81-run second-wicket partnership between Root (64) and Hales (56).
Both were guilty of not converting their fifties into hundreds despite looking good.
Buttler (61 not out) produced a late flourish to help his side get past the 300-run mark as the Kiwis applied the brakes at the right junctures.
All-rounder Ben Stokes played well for his 48 before getting out as Corey Anderson returned best figures of 3/55 while Adam Milne also took three wickets at the cost of 79 runs.
England - 310 (Root 64, Buttler 61*, Hales 56, Anderson 3-55, Milne 3-79)
New Zealand - 223 (Williamson 87, Plunkett 4-55)
Result - England won by 87 runs