The start of the Augments arc and the way the rest of the season was to unspool, "Borderland" is the moment Enterprise started to play heavily with franchise lore.
(Ha! Not that Lore, but close enough.)
In the first season, the series did a fair bit of this, and also in the second, but not in ways that fans had been expecting when news of the prequel concept first came down the pike (not that Pike!). The fourth season became a totally different experience, building on the heavy arc nature of the third but broken into smaller narratives.
"Borderland" is the first time Star Trek deals directly with the idea of the Eugenics War since "Space Seed" and the famed second movie (Wrath of Khan), though it was a major point in the Deep Space Nine episode "Doctor Bashir, I Presume?" It's a key piece of franchise history, though it's always been shrouded in mystery. The Augments arc, set a hundred years after the late '90s event (there have been many attempts to explain how we lived through something like that, or how the Voyager story "Future's End," set during this time, failed to mention it), deals with protoypical Khans suddenly unleashed, thanks to an ancestor of the man who created Data. (I don't normally reference actors in these capsules, but that's indeed Brent Spiner in these episodes.)
Also the first episode of the season to feature the Orions, but not the last.
franchise * series * essential * character
Memory Alpha summary.