If the target is already tapped at the time Plunge into Winter resolves, you'll still scry 1 and draw a card.
01-09-2023
wotc
You can cast Plunge into Winter without a target just to scry 1 and draw a card. However, if you do choose a target, and that target is illegal at the time Plunge into Winter tries to resolve, the spell won't resolve and none of its effects will happen. You won't scry, and you won't draw a card.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.