Alex finds herself in the midst of another possible threat in the next episode of "Quantico."

In the previous episode, Alex found out who the terrorist was. After tricking Alex into planting a piece of evidence on Ryan's computer and framing him as the terrorist, Drew turned out to be one of the thugs. Towards the end of the episode, Alex had to run off with Ryan's car, which contained the nuclear bomb.

In the upcoming episode, Alex remains on the run. And while she already knowns who the real terrorist is, she is putting herself out to take him down.

The official synopsis for the episode reads, "Alex learns someone she cared about at Quantico is not who they seemed to be, as she finds herself at the heart of another possible attack."

The promo teases that someone special to Alex might also be one of the terrorists, and it is not just Drew. It also reveals that the impending attack does not involve just another ordinary bomb but a lethal nuclear weapon, and Alex is driving it right into the center of New York.

According to showrunner Joshua Safran, the penultimate episode will disclose what piece of evidence Alex planted on Ryan's computer. Also, it will reveal more of the fate of Simon after he was taken by the terrorists.

"You will find out in the very next episode. I'm not going to say, but you will definitely know the answer," says Safran.

Titled "Closure," "Quantico" season 1 episode 21 airs on Sunday, May 8, 10:00 P.M. on ABC. Watch the promo below.