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There is a major turn of events left unexplained by the Marvel Cinematic Universe.
At the end of first Avengers movie (in 2012) we learned that Thor took the tesseract back to Asgard along Loki (as his prisoner).
Then, suddenly Avengers: Age of Ultron starts with the Hydra guy Baron Strucker sitting on Loki's scepter.
Can someone explain how and when did the scepter switch ownership?