On Quantico Season 3 Episode 6, Jocelyn's old handler has been taken, which stirs up memories for Jocelyn. We see her time undercover with The Widow where Frank was her handler. We see the accident that took away Jocelyn's hearing.
Frank was taken by Dante. Thanks to one of Dante's men, Leon, that the FBI picked up, the FBI is able to get into one of Dante's underground casinos.
Celine convinces the team that she is experienced enough to go undercover with Alex and Mike. Celine handles herself pretty well until she and Alex are taken by Dante. Dante also kidnaps Jocelyn by pretending to be Frank via text to lure them out.
It turns out that Dante has a fight club where the fighters have a knife duct-taped to their hand, and they have bags over their heads. This is why Jocelyn doesn't realize that she's fighting Celine and that she kills her.
Meanwhile, the rest of the team is frantically searching for Alex and Jocelyn. Deep and Shelby find Frank's case notes in a storage unit, and they uncover that Dante moves his money with ambulances.
They arrange a little heist and have one of the ambulance drivers take them straight to Dante's operation. They get there right as Alex, Jocelyn, and Frank are breaking out. Instead of arresting Dante, Jocelyn kills him.