The conclusion of a short arc that finally acknowledges that not every human was as excited about interstellar communities as the crew of the Enterprise, "Terra Prime" is one of the essential episodes of the series and franchise.
Mayweather's old flame from "Demons" turns into a knotty piece of the mess John Frederick Paxton has made, which Reed's Section 31 connection Harris helps untangle. But the crux of the episode involves the last best statement on Trip and T'Pol's relationship.
Something that'd been in development since the first season, when Trip formeda crush on the Vulcan officer, and intensified by the bond they shared in the tumultuous third season, Paxton unwittingly brings it to a close by choosing them to demonstrate the extremes of his belief that humans and aliens can't truly get along because they can't mate. (Well, Spock would beg to differ.) His machincations result in a baby that dies prematurely. The aftermath is crushing, one of the most purely emotional moments in franchise history (only Kirk losing both his friend and his son compare).
The episode has a fair bit of action, too, and developments that resonate with the rest of the season and series, including Archer delivering a speech many believe was shockingly missing from the finale, but the presence of that one moment elevates everything, ensuring the Terra Prime Duology a place among Star Trek classics.
Seriously, watch this one.
franchise * series * essential * character
Memory Alpha summary.