When SSA Derek Morgan was reunited with Ellie.
In the finale of season 5, the team investigated a serial killer that had been murdering families during the night for 26 years. Eventually the case led Derek and Detective Spicer, a local officer, to Spicer's home, where the killer was holding the officer's sister and daughter hostage. The killer had raped and severely beaten Spicer's sister and was holding a gun to his daughter, Ellie's, head when the two entered the room. The killer disarmed Derek and forced Spicer to his knees. Spicer begged Derek to save Ellie. Derek promised he would and the killer executed Spicer. In one of the most intense cliff-hangers, the season ended with Derek being knocked unconcious by the killer.
In the premiere episode of season 6, Derek wakes up to find that the killer has gotten away with Ellie in tow. The team worked hard to find them and eventually the episode ended in a face-off with Derek and the killer. When the dust settled, Derek left the scene to see Ellie safe, wrapped in a blanket outside. The two embrace in a tearful hug.
Season 6, Episode 1 “The Longest Night”