13 Huge Marvel Movie Mysteries That Are Still Unsolved

10. What Happened In Budapest

Again, we'll find out more about Black Widow's past - mostly before she left the service of her villainous former bosses, probably - when her solo movie is eventually released, but we may not get to see an answer to one of her most compelling questions.

What happened when she and Hawkeye were working together for SHIELD in Budapest?

In The Avengers, the mission - which Black Widow seemed to liken to the Battle Of New York, excitingly - is hinted at a couple of times between the pair, and while it could be no more than a gag, the fact that it's such a loaded mention makes it all the more frustrating that we don't know what went on there.