You may cast target instant or sorcery card from an opponent's graveyard without paying its mana cost.
If you want to cast the card, you cast it as part of the resolution of Memory Plunder. Timing restrictions based on the cardâ€™s type are ignored if itâ€™s a sorcery. Other casting restrictions are not (such as â€œCast [this card] only during combatâ€).
If you cast a card â€œwithout paying its mana cost,â€ you canâ€™t pay any alternative costs. On the other hand, if the card has additional costs (such as conspire), you may pay those.
If you are unable to cast the card (there are no legal targets for the spell, for example), nothing happens when Memory Plunder resolves, and the card remains in its ownerâ€™s graveyard.