In the second season finale, Jeremy was killed and brought back to life and it emerged that he can now see his dead ex-girlfriends Vicki (Kayla Ewell) and Anna (Malese Jow).
Plec has now revealed that the story behind Jeremy's new ability will still be a mystery when the third season begins.
"When we come back to see him at the beginning of the season, we'll see that he still hasn't had any success figuring out what it is they want, or understanding even what it is they are," she told Entertainment Weekly.
"We'll see that maybe he's been keeping this a secret, that nobody else is aware of what he's been going through, and that his behaviour has changed quite a bit and people are starting to wonder if he's perhaps getting back into some old bad habits. When in reality he's just trying to figure out why the hell he's being haunted by his dead girlfriends."
Plec promised that the show will have "fun" with Vicki and Anna, adding: "[We'll] start to really reveal the secrets of why they're here, what they want, and who else they have access to from our past... That's the beautiful part of our show. Just because someone meets their demise in our world, doesn't mean we'll never get to see them again."
Meanwhile, Williamson teased that viewers will begin to get answers about Jeremy's haunting in an October episode.
"Are they there for good?" he said. "Are they there for bad? It will once again be another twisty story. It's going to take a lot of turns and also involves some unlikely participants."
Ewell has previously admitted that she was "shocked" when she heard about the twist in the finale.
The Vampire Diaries will return for its third season on September 15 at 8/7c on The CW.
Watch the ending to the season two finale of The Vampire Diaries below: