If I activate Smoke Screen targeting an opponent's unit, but they use Retreat to return it to hand in response, will Smoke Screen retarget the remaining unit or fizzle?

Ruling: You cannot change targets once an effect is on the chain stack. Smoke Screen will still resolve but cannot find the original target, so nothing happens.

Nuances:

• Even though Smoke Screen doesn't affect anything, it still counts as being played for triggering other effects (like Student)