The earliest reference to the du Plessis family name was of Guillaume du Plessis, seigneur (lord) of Breux and seigneur of La Vervolière. He was married to Christian daughter of Hugues de Stanford and possibly Marguerite Countess of Warwick daughter of Henry Earl of Warwick.

Guillaume du Plessis is regarded as the first in the bloodline of Armand Jean du Plessis de Richelieu, Cardinal Richelieu.

In England:
An early mention is of Sir Hugh du Plessis (b. 1236 approx.) was the father of Christiane du Plessis (b.1263) of Stottesdon, Shropshire, England.

Warwick Castle was the seat of Margaret and her husband John de Plessis in the 1200's

In Canada:
It seems that the founding father of the du Plessis' (Duplessis) in Canada was Nicholas Gatineau-DuPlessis (b. 1627)

About Cardinal-Duc de Richelieu:

While both the South African and Canadian du Plessis' claim Armand Jean du Plessis as an ancestor this seems unlikely.

Very little seems to be known, on the Net in any event, about Canada's Nicholas Gatineau-DuPlessis, a soldier,and South Africa's Jean Prieur du Plessis's parentage is equally unrecorded. If either were related to the Duke proof of this would have surfaced by now (2008).

After the death of Armand Jean du Plessis de Richelieu in 1642 his title passed to his sister's grandson, who took the du Plessis de Richelieu name as well, as there were no male decendents at that time to inherit the name and title.

As the du Plessis surname had been around for over 400 years by that time there were probably hundreds, if not thousands, of du Plessis' in France at that time.

While it may be flattering to claim the Duke as an ancestor he would, at best, have been a distant one.