THE AVENGERS' GREATEST THREAT RETURNS! AND HE'S NOT ALONE... A rag-tag team of Mighty Avengers are thrust onto the front lines of SIEGE...with no one to lead them! And where's HANK PYM? Trapped in Pym-Space with his greatest enemy, his sinister cybernetic son, ULTRON! A war on two fronts, a team divided, and when it's over...not everyone will be coming back! Big threats, big changes, and the biggest Avengers surprise of them all revealed!