The unexpected enemy that emerges in The Avengers is Loki, Thor's brother and a powerful Asgardian villain who comes to earth with a plan to conquer and rule over humanity. He first acquires the Tesseract, a powerful energy source, from S.H.I.E.L.D. and uses it to open a portal through which he can bring an alien army to earth.
Nick Fury then assembles a team of superheroes to stop Loki and his army, including Iron Man (Tony Stark), Captain America (Steve Rogers), Thor (Thor Odinson), Hulk (Dr. Bruce Banner), Black Widow (Natasha Romanoff), and Hawkeye (Clint Barton). Despite their individual differences and conflicts, the Avengers must band together and use their unique skills and strengths to save the world from destruction.
The Avengers face numerous challenges and battles, including an attack on S.H.I.E.L.D.'s Helicarrier and a final showdown in New York City against Loki and the invading alien army. Along the way, they must also confront personal demons and sacrifices, such as Iron Man flying a nuclear bomb into the alien portal and the ultimate sacrifice made by one of the team members.
The Avengers ultimately succeed in saving the world and defeating Loki, but not without repercussions and consequences that will continue to affect them in future movies in the Marvel Cinematic Universe.