Dexter Morgan enters his fifth season in a state of profound shock and grief. The season begins in the immediate, raw aftermath of the murder of his wife, Rita. He is left to care for his infant son, Harrison, while also managing the grief of Rita’s children, Astor and Cody, who have gone to live with their grandparents. Dexter, emotionally stunted and unfamiliar with true mourning, finds himself numb and detached, struggling to connect with the very human pain surrounding him.
His work at Miami Metro is complicated by the arrival of a new homicide detective, Liddy, a corrupt former cop with a personal vendetta against Dexter’s sister, Deb. Deb herself is grappling with her own trauma from the previous season and a complicated relationship with her partner, Quinn, who is increasingly suspicious of Dexter. Quinn even hires Liddy to investigate him, putting Dexter under a dangerous new level of scrutiny.
Dexter’s path crosses with that of Lumen Pierce, a young woman who is abducted and viciously raped by a group of men. Dexter discovers her imprisoned and, in a moment of unexpected empathy, decides to rescue her instead of killing her captor as planned. Lumen, having survived her ordeal, is consumed by a burning desire for vengeance against the men who attacked her. She discovers Dexter’s secret and insists on helping him hunt them down.
A complex partnership forms. Dexter becomes a mentor to Lumen, channeling her rage into his code. They work together to identify and eliminate the members of the “Barrel Girls” gang, a ring of serial rapists and murderers led by the charismatic and monstrous Jordan Chase. Through this partnership, Dexter begins to feel a human connection he thought died with Rita, and Lumen finds a path to reclaim her power and confront her trauma.
Throughout the season, the investigation into the “Barrel Girls” killers puts Dexter and Lumen at constant risk of exposure. Jordan Chase, a motivational speaker who manipulates others to do his violent bidding, proves to be a cunning and formidable adversary. Meanwhile, Quinn and Liddy’s investigation closes in, leading to a tense confrontation where Dexter is forced to kill Liddy to protect his secret.
In the finale, after a brutal confrontation, Dexter and Lumen finally execute Jordan Chase. For a moment, it seems they have found a dark, peaceful equilibrium. However, Lumen’s need for vengeance, the “dark passenger” she developed, vanishes once her tormentors are dead. She realizes she cannot live with the killing anymore and must leave Miami, and Dexter, to heal and move on. They share a poignant, quiet goodbye.
The season concludes with Dexter alone again, but changed. He has truly grieved for Rita and experienced a deep, if doomed, connection with Lumen. He lets her go, understanding her need for a normal life, and returns to focusing on his son, Harrison. He carries on with his secret life, but now with a deeper understanding of loss and the fleeting possibility of shared darkness. The final shot shows him looking at a family photo, a solitary figure balancing his dual nature and his responsibilities as a father.