After the season-long arc of the Xindi attack in season 3 of Star Trek Enterprise, season 4 followed with several two and three-part stories. Thanks to a change in showrunner, season 4 saw more stories connected to TOS as a spiritual ancestor. One of the main threads of the final season was establishing long-term peaceful relations with not just the Vulcans, but Tellarites and Andorians as well. Together the four species founded the United Federation of Planets in 2161.
In 2155, several species arrive on Earth to discuss the potential of forming a Coalition of Planets. The Enterprise is recalled to assist and the crew are in attendance for Minister Nathan Samuels opening speech. They’re interrupted by a woman who presses a vial of hair into T’Pol’s hand before collapsing. The hair belongs to someone who has both T’Pol’s Vulcan and Tucker’s human DNA, which is impossible because T’Pol has never been pregnant. Archer vows that they will get to the bottom of this mystery.
Meanwhile, John Frederick Paxton runs his lunar mining colony he inherited from his father. He had acquired both Tucker’s and T’Pol’s DNA clandestinely and has their baby under medical care. He strongly believes this baby is an abomination, that the Vulcan DNA makes her humanity impure. The woman who found T’Pol at the coalition conference was a nurse who had worked for Paxton caring for the baby. The lunar colony is a hotbed for Terra Prime, a xenophobic group lead by Paxton that wants all the aliens to leave Earth for good and for humanity to revert back to isolationism.
The Enterprise crew is able to get Tucker and T’Pol work at the lunar mining company. It doesn’t take long for them to be discovered and detained by Paxton. They demand to see their child but he denies them for now. Instead he orders that the facility take off and warp 5 seconds to Mars. Paxton commandeers the array on Mars used for deflecting comets. He orders all the aliens of Earth in 24 hours or he will use the array to fire on any planet or ship that approaches Mars.
Starfleet scrambles to act, but Enterprise won’t fire on the mining facility, because Tucker and T’Pol are held hostage, and Paxton tapped into the Utopia colony. If he’s hit, he’ll take at least half the colony with him. Archer can’t justify the risk. Samuels mentions possibly replacing him with someone who can. The captain asks for a chance at a rescue mission as he and his crew come up with the plan of using one of the comets to hide their shuttle flight to the planet. It seems extraordinarily risky, but if it doesn’t work then Archer promises to fire on the array as ordered.
Tucker and T’Pol finally see their baby. Paxton forces Tucker to work on the array’s targeting scanners to improve them. He wants to be able to take out Starfleet Command without destroying San Francisco. Meanwhile, Travis pilots the shuttle with the captain, Lt Reed, and Doctor Phlox on board, leaving Hoshi in command of Enterprise. Despite an erratic flight plan, they’re able to land on the surface and sneak aboard the mining facility undetected. After a fight in the control room, with the array counting down and the window splintered, Archer barely manages to defeat Paxton in time before the array fires. Thanks to Tucker’s sabotage, the beam fires into San Francisco Bay instead of the original target of Starfleet Command.
Unfortunately, T’Pol and Tucker’s daughter is dying due to an imperfect cloning process and there is nothing Phlox can do. There’s enough time to get her back to Enterprise where T’Pol names her after Tucker’s sister, Elizabeth. Once Paxton and Terra Prime are apprehended, the conference to establish the Coalition of Planets resumes, this time with both Samuels and Archer giving speeches.
“Demons” and “Terra Prime” are the 20th and 21st episodes of season 4 of Enterprise. Thanks to low ratings, Paramount reduced the number of episodes in both seasons 3 and 4. This two-parter is not the series finale of Enterprise, but it should be as the last episode jumps ahead to 2161 with the founding of the Federation. However “These are the Voyages” is actually set in 2370 with Commander Riker from TNG using the Enterprise program on the holodeck to help make a decision. It was so poorly received by the fandom that it’s generally ignored. If some of the last scenes of “Terra Prime” were reworked and reordered so the show ended on Archer’s speech, then this would have been a great finale.