Much like Next Generation started its final season concluding the lackluster "Descent," Voyager returns to the Borg as a threat with nothing quite as much at stake as in earlier and later stories featuring the Collective.
Basically, Seven is reminded about another part of her past as a drone, a relationship she had and a whole underground scene that Janeway immediately recognizes as a potential weapon against the Borg. So she gets herself assimilated (sort of) to set up the cliffhanger.
It could have been better, and in this instance revealing a hidden part of a character's past seems a little forced, and hardly seems to really matter to Seven (personally, I would have at least ended this story with that love interest at least making recurring appearances during the final season), and the whole plan not a worthy comparison to either "Scorpion" or "Endgame."
But like I said, there was precedent in the franchise, just not a very good one. Forget "Descent," and just try to find out for yourself if "Unimatrix Zero" added something worthwhile to Voyager.
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Memory Alpha summary.