If you want to cast the card, you cast it as part of the resolution of Memory Plunder. Timing restrictions based on the card's type are ignored if it's a sorcery. Other casting restrictions are not (such as “Cast [this card] only during combat”).
01-05-2008
wotc
If you cast a card “without paying its mana cost,” you can't pay any alternative costs. On the other hand, if the card has additional costs (such as conspire), you may pay those.
01-05-2008
wotc
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 owner's graveyard.
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.