Someone has got a secret on “Once Upon a Time”! Mary Margaret (Ginnifer Goodwin) and David (Josh Dallas) can seemingly do no wrong. However, it looks like they’ve got a dark spot in their past that could potentially destroy their daughter Emma (Jennifer Morrison). Although it’s unclear what they’re hiding, fans will dig deeper into the mystery when Season 4’s episode 13, “Unforgiven,” is shown on ABC.

When viewers last left off on “Once Upon a Time,” villains Cruella de Vil (Victoria Smurfit) and Ursula (Merrin Dungey) had successfully managed to cross the town border of Storybrooke. Emma and Regina (Lana Parrilla) let the duo in after they helped defeat a hell beast and explained that they were there to look for their own happy endings. Mary Margaret and David suspected there was more to their arrival. And, according to the episode 13 synopsis, the Queens of Darkness could “threaten to reveal a secret they thought was long buried.”

Although Emma let the “former villains” into Storybrooke, she’s not 100 percent certain Cruella and Ursula are on the up-and-up. She’ll begin to look into the pair, and catch on that her “friends and family are keeping something from her.”

Elsewhere, Regina and Henry (Jared S. Gilmore) will dig deeper into the mystery of the author. They learned last week from Mother Superior (Keegan Connor Tracy) that the Author and the Sorcerer are two different people, but they hit a dead end. To get a new lead, the mother-and-son team will turn to Pinocchio (Jakob Davies) and Marco (Tony Amendola).

Viewers will remember that Pinocchio, aka August W. Booth (Eion Bailey), was linked to the fairy-tale book when he first arrived in town. He owned a typewriter and added his story to Henry’s book. It’s unclear how much he knew about the book -- but Regina and Henry are going to try and get to the bottom of it. The only problem is that August turned to wood when magic returned to Storybrooke.

Finally, fans can look forward to another flashback to the Enchanted Forest. In the flashback, Snow and Charming will “cross paths with the Queens of Darkness as the threat of the Evil Queen’s Dark Curse hangs over all of them.” 

Does this mean that “Once Upon a Time” fans will learn about the secret being concealed by Mary Margaret and David? “Once Upon a Time,” Season 4, episode 13, will be shown on ABC Sunday, March 8, at 8 p.m. EDT. Watch sneak-peek videos of “Unforgiven” below: