Scorpion

One of the most pivotal episodes in Voyager history. The Enterprise is finally in Borg space but they have found a corridor through it that seems to be relatively free of them. As they make their way through it, the realise that there's a very good reason that the Borg are staying away and that reason is known as Species 8472. Species 8472 is unbelievably enough, more powerful and malevolent than the Borg themselves.

Janeway then makes a deal with the devil a.k.a. The Borg. They will help the Borg develop a biological weapon against Species 8472 in exchange for which Voyager will be given safe passage through Borg space. Of course, whenever the Borg is involved, things are never quite as straightforward. The end result is one of the biggest changes seen to the series since it's inception - the introduction of the character 'Seven of Nine' or simply Seven.