A blue box in Shield

What would happen if one day a blue box from the 60's just appeared in the middle of the Shield Bunker?
They have no idea where does it come from or how.
Meanwhile Clara and the Doctor just came up in a room full of people pointing them with guns and a grip of heroes defending from them. They don't know how they landed there. But it gets weirder when the TARDIS just disappears.
What would happen if they discover they're in great danger of and mystic alien force? Will The Avengers and Clara and the Doctor will be able to stop it before it's too late?

Okay so again, my first idiom isn't English so if I have some mistakes, make me know
Set after "Avengers" and Captain America: The Winter Soldier and Thor:A dark world and
After "The Name of the Doctor" and "The Day of the Doctor" but before "The time of the Doctor"
Some spoilers from the Name of the Doctor (like a clara echo dating Bucky as Connie)