Stuck in the ocean miles from shore, Dexter is rescued by an old boat carrying illegal immigrants to the US. Once ashore, he heads home hoping to figure out where and when Travis next move. Travis has holed up in a small house and has killed the occupants. While he's away, the police show up and Dexter has something of a surprise waiting for him. Dexter makes it to son Harrison's school recital only to have Travis take the child. Meanwhile, Debra has a plan to catch Travis and gets LaGuerta's support for her plan. Debra also comes to terms with her feelings for Dexter. In his final confrontation with Travis, Dexter has him tied down on his execution table. Someone walks in just as he's committing the deed.

Dexter finds himself at the yacht where Travis and his accomplices were preparing the next tableau, "Wormwood". They quickly realize that Travis is planning a gas attack. Worried that Batista has not been seen since the previous day, Quinn sets off to find him. Travis meanwhile sends Beth Dorsey to Miami Metro homicide with a canister of deadly gas and orders her to meet with Lt. Morgan. Deb has dinner with Deputy Chief Matthews and discusses the case of the dead call girl. The next day Deb realizes that Laguerta is very adept at office politics.

Now realizing the error he has made, Dexter redoubles his efforts to track down the doomsday killer and eliminate him. He plants evidence in the church to keep the police off the scent and sets out to find DDK. He manages to track down Holly, the woman Travis freed, but too late. From the evidence at the crime scene, Dexter believes he knows what DDK will strike with next, though he has no idea where. With Quinn hung over and not at work, Batista sets off to interview a person of interest which may have major ramifications for him. Meanwhile, Deb goes through the evidence from the dead call girl case and now knows who was with her and who Laguerta is protecting. Louis the crime lab intern is more than a little deflated when Dexter slams his new computer game.

Dexter tries to stay one step ahead of the Homicide Division concerning the Doomsday Killers, while he finds help from an unlikely source; Debra uncovers something unexpected about herself during her therapy sessions.

Dexter, using what he's learned from Brother Sam, comes a step closer to the Doomsday killers, specifically Travis; Deb and LaGuerta go head-to-head after a call girl ends up dead.

Miami Metro gets closer to Gellar and Travis thanks to the new intern. Meanwhile Dexter deals with faces from the past. Travis tries to move on from Gellar.

Dexter investigates who shot Brother Sam. Meanwhile, Homicide interrogates the woman who Quinn slept with who is involved in the Doomsday Killer case.

With the help of his newfound friend Brother Sam, Dexter wonders if there is light within him to counter the darkness, while the search for the Doomsday Killers takes him in a new direction; Batista and Quinn pay a visit to the university where Professor Gellar taught; due to departmental regulations following the shooting, Debra is forced to begin therapy.

The team investigates murders that have a religious undertone to them.

Dexter faces a serial killer from his past; Debra has problems with her new position and Travis with Gellar think of their next move.

In order to solve the mysterious murders, Homicide asks for the help of Brother Sam, who Dexter knows used to be a serial killer. Debra has two proposals, one personal and one job-related.

Dexter uses his 20 year high school reunion as a chance to hunt the prom king. It's going fine until he is called to the scene of a vicious faith-based murder leaving him with questions about what he wants to pass on to Harrison. Debra who is finally in a good place with her relationship with Quinn and her job becomes an unexpected hero.