Loki, The Asgardian, had a deal with The Other, the leader of the alien race, Chitauri. The Other promises Chitauri army to Loki to help him conquer the Earth as the exchange of Tesseract, a powerful energy from unknown source.
Meanwhile on the earth, Nick Fury, the director of S.H.I.E.L.D, was notified that Tesseract was active. Even Dr. Erik Selvig didn't know how it became active.
Suddenly Tesseract opened a portal, and Loki appeared.
Loki then attacked the S.H.I.E.L.D. base. He also influenced Clint Barton/Hawkeye, and Dr. Erik Selvig with his power, so they become Loki's minion.