ST:TNG:75 - "The Best Of Both Worlds, Part II" (Stardate: 44001.4) - this is the 1st episode of the 4th season of Star Trek: The Next Generation.
The Borg are back! The last time they were encountered was 7000 light years away in the 2nd season episode "Q Who?", with Starfleet thinking they'd have more time to prepare. Continuing off from part I (in the first cliffhanger episode in TNG), the Enterprise, under a newly promoted Commander Riker with Shelby as his Number One, tries to rescue Picard who has been assimilated by the Borg, Picard now renamed Locutus.
Can the Enterprise rescue Picard from the Borg, with Riker working together rather than against Shelby, thereby saving the Federation from its most dangerous threat? Watch this part II of an exciting two part episode - it is the cliffhanger that you were waiting for and conclusive evidence that The Next Generation can indeed hold its own (from under the shadow of the Original Series).
Trivia note: Although the battle at Worf 359 is only mentioned here, we will get a chance to see it in the Deep Space Nine pilot episode "Emissary". Also the Enterprise separates again in one of the few times of the series. And Whoopi Goldberg makes another appearance as Guinan.