In a lot of ways, this is a sequel to "Carbon Creek" from earlier in the season. It's a story that Archer tells T'Pol, much as the reverse is true of the other episode. Anyone who still believes that Enterprise's second season was pretty random will hopefully have one less reason to think so.
It's also a love letter to NASA, something Star Trek had done before in Voyager's "One Small Step." At a time when the US space program was already in a decline and only a few months after the Columbia tragedy, it perhaps is appropriate that Star Trek did so again.
It's also a rare flashback in franchise lore. Deep Space Nine did it with Odo and Kira, but for the most part the past was at best something characters only talked about. "First Flight" presents Archer's efforts to get his father's warp engine approved for deep space travel, as well as his rivalry with A.G. Robinson, whose death in the present is the impetus for the occasion.
In a lot of ways, it's the episode fans had been waiting for from the start, dealing with Vulcans actively discouraging human technological advances. Archer meets Trip for the first time during the depicted events, so it's an origin story in several regards. Anyone still wondering just how perfectly the second season treated Trip must also view this episode as proof. He serves as an excellent counterpoint to Archer, even though they eventually conspire in the same test flight that gets Starfleet the mandate to reach "Broken Bow" (the series pilot), even if it nearly grounds the program in the process.
If you view only one episode from the season, "First Flight" should probably be that episode. It's a perfect example of Enterprise at its finest, and is a worthy induction into the new classics of the franchise.
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Memory Alpha summary.