The Avengers isn't exactly a sequel, but you will get more out of the experience if you see the films that lead up to this point: Iron Man (2008), The Incredible Hulk (2008), Iron Man 2 (2010), Thor (2011), and Captain America: The First Avenger (2011). These films serve to give background to the characters, and also give you a better understanding of the story.
The plot of The Avengers is rooted very strongly in plot lines set up in the previous films. The sibling rivalry between Thor (Chris Hemsworth) and the film's primary villain, Loki (Tom Hiddleston), continues. The Tesseract device, an object with unlimited power first seen in Captain America, is stolen by Loki, and with it he plans to enslave all of mankind.
Standing in his way are Earth's Mightiest Heroes, who spend as much time fighting each other as they do their mutual enemies. The group finally learns to work together, and with their combined might they attempt to stop Loki's army.
The plot really isn't very original at all, but that works to the film's benefit. Rather than being story heavy, the film focuses on giving the audience all the super hero action they expected to see. The Avengers clearly was made with one goal in mind: to be fun from beginning to end.
Mission accomplished; there isn't a single dull moment to be had. While the story may be rather simple, it is perfect for moving the action along, while also creating a necessary level of suspense for the audience. I can't emphasize this enough — it's a typical comic book storyline, but it fits the tone of The Avengers perfectly.
Director Joss Whedon should be commended for making one of the most well-crafted superhero films of all time. This is an excellent-looking film, and its visual perfection goes far beyond the dazzling special effects. Whedon, along with cinematographer Seamus McGarvey, display some very creative camera work. This gives some indication that the filmmakers really cared about this movie a lot, and weren't just interested in a commercial success — a lot of love went into bringing The Avengers to life, and it absolutely shows.