Starfleet and the Borg clash in what becomes known as the Battle of Wolf 359, or more appropriately, the Massacre of Wolf 359. 39 of the 40 ships sent by Starfleet to stop the Borg are destroyed, costing the Federation 11,000 lives.
Data and Worf terminate five Borg drones during their attempt to rescue the assimilated Captain Picard.
As the Borg cube closes in on Earth, it is approached by three probes or possibly scout ships. The cube destroys them all with a volley of plasma fire.
The Borg are defeated when Data hacks into their neural network and commands the drones aboard the cube to enter their regeneration cycle, putting them to sleep. The conflicting commands in the collective network trigger a self-destruct sequence aboard the cube and it explodes in Earth orbit.
Early in the episode, Cardassian commander Gul Macet informs Captain Picard that a Starfleet ship, the USS Phoenix, has destroyed a Cardassian science station.
Enterprise uses stellar maps to monitor the activities of rogue Starfleet vessel, USS Phoenix. The Phoenix attacks a Cardassian supply ship, destroying it and killing its crew of 50. A Cardassian warship intercepted the Phoenix but was also destroyed, with 600 crew lost.
Chief O'Brien reminisces about the Federation-Cardassian war and goes back to the time of the Setlik III massacre where over 100 civilians were killed by Cardassian troops.
O'Brien tells Glinn Daro about the first time he killed. During Setlik III, he was tackled by a Cardassian soldier and a struggle ensued. A woman threw O'Brien a phaser set at maximum and he instinctively fired, vaporising the Cardassian.
During the episode, the Enterprise is studying a dark-matter nebula, but the dark-matter causes various areas of the ship to phase out of normal space, resulting in objects passing through their placements and even momentary hull breaches. In one instance, a crewman is killed when she phases halfway through the floor.