Part one: As the mission starts, you will se a guard in front of you. Eliminate him and climb down to where he was. Look to your right: there is a box with a silenced 9mm if you need ammo. Then go left and climb down the stairs, fallowing the scientist. Always remember to keep a reasonnable distance between you and him. When you get to the bottom of the stairs, take cover in the alcove on other side of the tunnel. Soon, you will see a guard passing by you. Eliminate him quickly. Keep going after the scientist.