In the second season finale, Jeremy (Steven R McQueen) was brought back to life after being shot and was able to see his dead girlfriends Anna (Malese Jow) and Vicki (Kayla Ewell).
However, Plec has now revealed that Matt (Zach Roerig) will also be able to see Vicki, his late sister.
"Matt is struggling with the lingering [supernatural] element in his dead sister, who's popping up to Jeremy," she told TV Line.
"We'll get to see Matt struggle with, 'My dead sister is paying visits to my ex-girlfriend's brother, who used to sleep with my dead sister'."
Plec has previously revealed that Anna and Vicki have a "master plan", but has explained that Jeremy will not understand what is happening when the show returns.
The third season of The Vampire Diaries will premiere on The CW on September 15.
Watch the latest trailer for The Vampire Diaries, which includes a clip from season three, below: