Three issues ago X-Men got its first crossover with the rest of the Marvel universe. X-Men #9 takes this up a notch, featuring a guest appearance of the Avengers, whose ongoing series launched in the same month and year as X-Men. It features the classical line-up of Captain America, Iron Man, Thor, Giant Man and the Wasp.
The comic starts with the X-Men travelling to Europe on a ship. This anachronism isn't remarked on, but appears to exist solely in order to give Cyclops a chance to blast a wild iceberg out from the path of the ship. They've been summoned to act as backup for Professor X, who has tracked down his previously unmentioned nemesis Lucifer. The Avengers also have arrived, having detected Lucifer. Possibly the funniest moment in the series so far happens when a passer-by gets spooked out by the Avengers' strange dress, and drives away quickly to stop at the next group of apparently normal people he sees: the X-Men.
Being a superhero comic, there are certain narrative conventions to obey. So the Avengers and X-Men end up in a fight - in this case ostensibly because the Avengers would interfere with dealing Lucifer without all the facts. Thankfully, after a few pages, Professor X is able to calmly describe the situation telepathically to Thor, and they part as friends.
The comic makes a really big deal about how young the X-Men - all still teens - are compared to the Avengers, something that will become less apparent as the decades wear on and the X-Men are allowed to grow up, while the Avengers stay broadly static. Oh, and one other thing.