Gladbach hit Shakhtar for six in Champions League
As well as Plea's goals, a Valeriy Bondar own goal and counters from Ramy Bensebaini and captain Lars Stindl gave Gladbach their finest European victory since their 1970s glory days.
After being pegged back to draw 2-2 in their first two outings in Group B, away to Inter Milan and at home to Real Madrid, the victory moves Marco Rose's Gladbach to the top of the section, one point above Shakhtar.
It is the Ukrainian champions worst defeat in a home game in Europe, although they are currently hosting matches in Kiev having been exiled from Donetsk since 2014 when conflict broke out in the east of the country.
French forward Plea met Stefan Lainer's low cross with a first-time finish to put the visitors in front in the eighth minute, before a horror show from Bondar allowed them to double their lead. First his miskick gifted the ball to Plea, and then Christoph Kramer's shot took a huge deflection off the Shakhtar defender on its way in.
Plea's superb strike into the top-left corner made it 3-0 in the 26th minute, and Algerian defender Bensebaini got the fourth from close range just before the break. Stindl's goal to make it 5-0 arrived with 11 minutes left, and there was still time for Plea to make it six after being played in by Marcus Thuram for a goal that was given after a VAR review confirmed he was onside.
The result is a bitter setback for Shakhtar after they began their campaign by beating Real Madrid 3-2 in Spain and then held Inter last week. Real and Inter meet in Madrid later on Tuesday.