Forum:Mystical Space Typhoon vs Heart of the Underdog

I have Heart of the Underdog and Veil of Darkness, I am chaining multiple draws when off of one of the Veil chains I reveal and discard a Leg of Exodia, my opponent realizing what is going on, activates Mystical Space Typhoon targeting Heart of the Underdog. Does he have a timing he is allowed to do this, and further more does it prevent the chains from continuing?