Pointed Discussion

Im curious how it would hold up in a vintage format. Too slow? Too unreliable? Any potential?

This art rocks. I love how the merfolk appears to be stealing a burn spell.
This card is for Timmys. When it works out, it will create classic moments. My favorite way of winning a game is by using my opponent's cards because it's always different and creates great memories. And in casual, I don't think there's any reason not to run this. It will almost always have a target and can become quite versatile against instant/sorcery-heavy decks.

if they ever do make a pirates versus ninja deck, this is so ninja

Is it possible to 'steal' and X spell with this card, or would that be a waste of time?

Cool card but too situational.

No, you can not use this on an X spell and hope to get anything. It's a waste of time.

Since you don't pay the mana cost, X will always equal 0.

Would go great in a mill deck sideboard. That way when you figure out what kind of stuff they have, you can decide if it would be nice to mill their Cruel Ultimatum and then use it against them... or if their best spell sucks, you probably won anyways.

What I find really fun is using Mirari to copy Memory Plunder, steal my opponent's Memory Plunder, copying that one, taking Archive Trap or anyother large scale mill spell/kill spell, copying that spell and then watching my opponent stare in wonder at their empty library/empty board. That combo is really, really hard to pull off, but the looks from your friends are well worth the trouble.

@TwoStars,

The first ruling under the card details should answer your question.

If I play this spell targeting my opponents Wrath of God during an opponents combat step, will I be able to play Wrath of God? Or will its sorcery restriction stop me from playing it?

EDIT: My bad.

Considering a large number of vintage decks have the same blue backbone (Ancetral recall, Time Walk, Brainstorm/Ponder/Preordain etc) this card takes a lot of the situation out of situational.
However, four mana is a lot to just recast their Ancestral Recall. On top of that, it's colored mana that can't be paid with off-colored moxen or Mana crypt/vault.
Overall, cards in vintage that are hardcast with a CMC >=4 usually win the game (Jace, Tendrils, Gifts, Lodestone, mind's desire, ad nauseum etc) and all these cards find themselves with some kind of acceleration (Ritual, Mana drain, workshop). It's pretty good sideboard against Combo or control mirros, however. A nice 1-2 punch to duress their mind's desire, accelerate, and then Memory Plunder it for your own win.
Other formats, as Revelation stated, have a lot more archetypes and are too situational.

This'll be fantastic in the Sanity Grinding deck I want to make.

Oh, you milled that? I think I like it. Could I borrow it? Thanks.

fixed Yawgmoth's Will :)

sort of.

Opponent pays 10 mana for time stretch. You cast memory plunder for 4 mana, copying time stretch, you take the extra turns before they do ;)
*Trollface*

You're a mean one, Mr. Grensch.
You really are a heel.
You're as cuddly as a cactus,
You're as charming as an eel,
Mr. Grennnnsch!
You're a bad banana with a greasy black peel!!!!

Flavor text: Dafuq does that even mean?

Wow this card is insanity. EDH Gold.