fire towards mouse pointer

I have a plane, and i need a script so a the missiles will always fire to where the mouse pointer is. How would i do this.? The projectiles fire from where the missiles are on the plane, and I want them to fire in the direction the cursor is pointing. Plane

Using Camera.ScreenPointToRay and doing a raycast will get you the point on the plane. All you need to do then is point - firingPosition and you will have the direction in which to fire your missile.