Vader chases Luke through the facility, operating his light saber in full-on baseball bat mode and using space magic to throw a bunch of random shit at the young Rebel, until finally cornering Luke at the end of a bridge and chopping his goddamn hand off. Vader then reveals himself to be Luke's supposedly dead Jedi father, to which Luke responds with mewling shrieks of terror-sadness and then leaps into the cavernous pit below. Thanks for coming, kids! The next movie will be out in a few years!
But in the Game ...
When they sat down to make the NES game in the early 1990s, they decided they were going to get rid of all this depressing shit and write the movie Lucas should have made. So, instead of getting tossed around like a child, Luke arrives at Cloud City to greet Darth Vader with a howling explosion of furious ass-whoop. He sprints down the spindly chasm bridge and pummels the Sith Lord like a jazz drummer cracking open a pinata with a pair of nightsticks, until Vader simply collapses under the weight of his own embarrassing failure and falls off the fucking bridge.
He just shit in his robot suit, and now he has to smell it all the way down.
However, the greater departure from the film's storyline comes after Luke beats Vader in 10 seconds like he was late for a meeting. Once Vader is defeated and sent hurtling down the inexplicable sky abyss/twinkling future slide, he delivers this message:
"Next time, Skywalker ... NEXT TIME!"