Archer receives a surprise visit from Daniels, the mysterious time traveling operative who has aided him in the past. Daniels tells Archer that three Xindi-Reptilians have somehow traveled back in time 150 years to Detroit, Michigan. He wants to send Archer and T'Pol back in time to investigate. He also gives Archer temporal tags to bring back anything that doesn't belong in the 21st century. After disguising them in clothing from the proper time period, Archer and T'Pol successfully travel 150 years in the past, arriving at night on a Detroit City street.
Using a hand scanner, they find a car to use and take cash from an ATM. T'Pol uses her scanner to search for Xindi bio-signs, and the duo winds up at an abandoned factory on Carpenter Street. T'Pol also notes that there are several humans in the building. T'Pol and Archer search the area and detect a neutronic power source. They also observe a man, Loomis, arriving. When he emerges from the building, Archer and T'Pol follow him to his apartment and apprehend him.
Loomis assumes they are police officers, so they question him and manage to learn more about the Xindi operation. Loomis is a blood bank worker who has been procuring human victims for the Xindi, one for each blood type. He renders them unconscious and delivers them for a fee. He has no idea as to the exact nature of his "employers," though, as they always appear in the shadows. Loomis tells them that he has delivered six of the eight blood types and only needs two more — B-negative and AB-positive. Hearing this, Archer devises a plan — he will pose as the B-negative victim. Loomis agrees, thinking that T'Pol and Archer will cut him a deal for helping them.
Loomis takes Archer — who is pretending to be unconscious — to the factory. Once the Xindi take his blood and leave him alone, he explores the operation and contacts T'Pol with his findings. Their worst suspicions are confirmed: the Xindi appear to be constructing some kind of bio-weapon using the human blood samples. Apparently, they have traveled through time to use the past as a hiding place for the weapon. Archer considers taking out their bio-reactor, but realizes that if he does, he will spread the virus out into the city. T'Pol suggests he try finding and destroying their temporal beacon, to prevent them from returning to the future. Archer succeeds in locating and taking out the temporal beacon and kills one of the Xindi. The other two escape, taking a canister of the deadly virus with them.
Meanwhile, T'Pol subdues Loomis after he spots the Reptilians and tries to attack her. Archer takes down one of the Xindi, and he and T'Pol chase the remaining Reptilian to the rooftops. A fire fight ensues, but Archer manages to corner the Xindi just in time and stops him from releasing the virus. Archer and T'Pol then use the temporal tags to take the three Xindi bodies and the Xindi equipment back to Enterprise.
Back in Detroit, Loomis awakes to find himself being arrested by the police, who have connected him to the six unconscious bodies in the abandoned factory. Loomis tries to explain about the "lizard people" and their "ray-guns," but to no avail.