One of the pivotal episodes of Star Trek. After this episode nothing in Starfleet will stay the same.
Q returns and offers a helping hand, and wants to work as a crew member. Picard refuses – and sends the enterprise into a distant region of space. It will take 2,5 years to get back to earth. The Enterprise encounters the Borg for the first time.
The Borg Collective is different from everything the Federation has ever encountered. You cannot communicate with an individual borg only with the collective. The borg is directly connected to the ship and the ship can self regenerate and the collective wants to assimilate all technologies. (Later they change that to also assimilate biological distinctiveness). The terror of the Borg is further intensified as the Borg never communicate and only show one superior technology and simply overpower the enterprise. They repair the damage and adapt and the enterprise cannot damage the ship anymore. Picard has to ask Q for help to survive and he accepts. Things are back to normal for the moment. – It will take another 3 years until we see the cube again.
There is a lot of debate if Q helped the Federation to survive by introducing the Federation to the Borg early on. Or if Q actually made the Borg aware of the Federation causing immense chaos and destruction. I would say Q was helping the Federation. The Federation was already aware of the destruction of the colonies in the neutral zone – that seemed similar to the destruction they scanned on a planet nearby.
Sadly one of the biggest mysteries they introduced in this episode is Guinan not being that what she seems to be and has apparently powers that can rival a Q – was never explored further…