The Originals would follow the character of Klaus (Joseph Morgan) as he returns to his old haunt in the French Quarter of New Orleans.
"The idea is that a turn of events in Mystic Falls sends Klaus to New Orleans to solve a mystery," Plec explained to Entertainment Weekly.
"He gets word that some people are making a move against him that might concern him. So he heads to New Orleans to try to figure who's plotting against him and what they're up to."
The spinoff is yet to be picked up to series, but Plec insisted that the project would not "detract" from its parent show if it receives a full commission.
"It all came together in a way that we finally felt would not detract from our most important baby right now, The Vampire Diaries, and would give us a chance to expand the universe," she said.
Phoebe Tonkin would also play werewolf Hayley in The Originals though Plec added that she would prefer the circumstances surrounding the character's involvement to remain a "big mystery".
However, the showrunner did reveal that she wants Vampire Diaries actor Daniel Gillies to reprise his role of Elijah in the new series.
"It's only a matter of whether in Daniel Gillies' world there is an interest... but I can hope the answer is yes," she said.
The Vampire Diaries continues on The CW in the US and airs on ITV2 in the UK.
> CW boss on Vampire Diaries spinoff: 'Klaus is a great character'
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