1st look at 'Grey's Anatomy' season 21 features a slap and familiar faces: Watch here

Tensions are running high, and some familiar faces are back in the first look at "Grey's Anatomy" season 21.

The teaser features the return of actress Kali Rocha, who played Sydney Heron in guest and recurring capacity from season 2 to 4, and Jesse Wiliams as the fan-favorite doc Jackson Avery.

Season 21 picks up seemingly where we last left our characters, with Meredith (Ellen Pompeo) at odds with Catherine Fox (Debbie Allen). In last season's finale, Meredith quit Grey Sloan after publishing her Alzheimer's research -- exactly what Catherine told her not to do.

In a heated moment at the end of the teaser, Catherine threatens to sue Meredith -- though she is clearly unshaken. "Your lawyers know where to find me," Meredith retorts.

Catherine confronts Miranda Bailey (Chandra Wilson) in another scene, telling her, "You think you're God's gift to this hospital? You're just another doctor," resulting in Miranda slapping Catherine across her face.

In less dramatic scenes, Rocha's Sydney is ready to lead the residents with her "heal with love" mentality, which is hilariously met with stunned faces, and Meredith welcomes Williams' Jackson back, though he doesn't exactly seem thrilled to be there.

"Grey's Anatomy" season 21 premieres Thursday, Sept. 26, at 10 p.m. ET on ABC.