In the second season finale, Jeremy (Steven R McQueen) discovered that he could see his late girlfriends after he was brought back from the dead.
One of the women that returned was Vicki (Kayla Ewell), the late sister of Matt (Zach Roerig).
Plec has now told Entertainment Weekly that Vicki's arrival will have a big impact on Matt.
"Matt is one of the first people to learn Jeremy's secret, and one of the loves of Jeremy's life visiting him from beyond the grave is Matt's sister Vicki, who he never really had complete closure with," she said.
"So he is going to have, with Jeremy, an emotional journey in this supernatural world as he learns that his sister's out there and contactable and tries to convince Jeremy to help him get the closure that he's needed."
Plec recently claimed that the women Jeremy can see have a "master plan", and has now said that more about their motivations will be revealed as the season continues.
"We're going to have some fun with Vicki and Anna and start to really reveal the secrets of why they're here, what they want, and who else they have access to from our past," she teased.
The Vampire Diaries will return with its third season on September 15 on The CW.
> Vampire Diaries exec hints at Matt struggles
Watch a trailer for the new season of The Vampire Diaries below: