“Grey’s Anatomy” left our mouths agape when Callie (Sara Ramirez) revealed who her mystery lover was in the final Season 12 scene of episode 4. And while the arrival of Callie’s beau was hard on Arizona (Jessica Capshaw), who was chugging wine at the thought of meeting her ex’s new girlfriend, the debut of Penny (Samantha Sloyan) was even more difficult for Meredith (Ellen Pompeo) – which fans will see in episode 5 of the hit ABC series.

When Mer met Penny she immediately had a flashback to the night of Derek’s (Patrick Dempsey) death. As it turned out, Penny was the surgeon who was assigned Derek’s case during the Season 11 installment and failed to keep him alive.

“It was my job to save him,” Mer could hear Penny saying from the night she became a widow.

During the shocking Season 11 episode, Penny blamed herself for Derek’s death, telling Mer she wasn’t “good enough” to keep Derek alive. But Mer responded unexpectedly. Instead of lashing out, she told the doctor to “be better.”

“He was the one that died on your watch. He will haunt you – the hard ones always do,” she said. But little did Mer know that Penny’s face would haunt her, too – just like she had warned the interns while advising them on how to announce the death of a patient.

“You are changing this person’s life forever. You are responsible for this moment,” Meredith told them in episode 4, explaining that their face will be burned into the memory of the families for a lifetime. “Recognize the importance of your role. And respect that that person’s pain is the biggest thing in the room. Got it?”

Meredith’s friends will learn more about Penny’s surprising identity when Arizona nervously pries during the dinner party in episode 5. But that won’t be the only drama set to go down in the upcoming installment, titled “Guess Who’s Coming To Dinner.” “In the midst of the sisters’ dinner party, Maggie (Kelly McCreary) ditches her cooking duties to rush to the hospital,” the synopsis says, adding that April (Sarah Drew) will take the chef's place to “avoid Jackson (Jesse Williams).”

“Grey’s Anatomy” airs Thursday night’s at 8 p.m. EDT on ABC.