The second season ends with a bang, concluding the crew's efforts to figure out how they fit into the grander scheme by letting them know that they have a long way to go. This is accomplished by kicking off the Xindi arc with a giant swash of Florida being eradicated, and also the last efforts by Duras to gain revenge on Archer, concluding another arc begun in "Judgment."
In a lot of ways, this is Enterprise being Deep Space Nine, having the series steeped in its own mythology. Next Generation started this with "Redemption," which also featured Klingons, and certainly "The Best of Both Worlds," still the most famous but not first appearance of the Borg. Contrary to how much of the preceding season had played out, there's a definite connection to other episodes (another irony of the perception of the season, given that even in the early episodes, there was a link between "Minefield" and "Dead Stop," and Trip references plenty of things we've seen in "Dawn").
Another notable feature of "The Expanse" is its inclusion of the Temporal Cold War arc, which even at this point seems to have been altered from its original conception. Future Guy makes his last appearance of the series, undeniably aiding Archer, as previously suggested for his motives in "Cold Front" from the first season. Rather than a black and white, Silik-is-the-bad-guy/Daniels-is-the-good-guy interpretation that some episodes might have argued (notably "Shockwave," which ambiguously ended the first season and began this one), we have reason to believe after "The Expanse" that it benefits plenty of factions for events to play out as they historically did, rather than be altered.
Which in itself suggests that the whole third season could be redefined as the definitive Temporal Cold War story, even though the familiar players from other stories in the arc are mostly absent (Daniels appears a few times). But that's an argument for another time.
Even if you don't care about that arc, there's plenty to enjoy about the episode. Trip loses his sister in the attack, which is the last push the season gives him for everything that would come, the material that would truly build his enduring legacy. The Xindi crisis itself reflects the 9/11 attacks that had just occurred when the series began, marking the first time Star Trek used contemporary events to inspire an ongoing story (unless you count the other Cold War and the Klingons).
If you don't particularly want to watch this one for all the interseries continuity, it can still be enjoyed as a standout Star Trek adventure.
franchise * series * essential * character
Memory Alpha summary.