Back Off

What happens when my empowered Gangplank, Naval is moved by my opponent's Blast Cone's play effect and they exhaust Blast Cone to stun Gangplank? Will he be stunned? What happens if I then play Back Off choosing my Gangplank? Will he get +3 [M] this turn?

Blast Cone will stun Gangplank. If you play Back Off choosing your stunned Gangplank, he will not receive +3 [M] this turn.

Blast Cone's triggered ability that stuns units doesn't choose them, so Gangplank's replacement effect can't apply to that event. Back Off played on a stunned Gangplank does not actually perform the stun game action, because Gangplank is already stunned. It's legal to play Back Off, but the instruction will be negated. In order for Gangplank's replacement effect to replace an event of him being stunned, the stun game action must actually be performed. Similarly, if an effect that said "Friendly units can't be stunned" were active with a friendly empowered Gangplank, Naval is on the board, a stun effect that is played choosing Gangplank wouldn't generate an event that Gangplank could replace, because no stun game action is performed.

Relevant Rules: 423.1.a.1.

135.2.b.7. Instructions of a spell or ability may reference actions taken during Finalization. They will do so by checking whether "you" performed those game actions. These instructions check whether the game action was performed, not by whom. If another player gains control of the spell or ability after the game action was performed during finalization, it will still be considered true for the purposes of these instructions. Example: Rampage is a spell that reads "As you play this, you may pay [C] as an additional cost." and "Choose a friendly unit and an enemy unit. If you paid the additional cost, give the friendly unit +2 [Might] this turn. They deal damage equal to their Mights to each other." If another player gains control of Rampage, although they are not the one who paid the cost, the "if you paid the additional cost," condition will still be fulfilled. Example: Back Off is a spell that reads in part "If you played this from your hand, draw 1." On resolution, Back Off checks if it was played from hand. If another player gained control of Back Off, as long as the card was played from hand, the condition will be true and the controller of the spell will draw 1.