Everything about the original spell is copied, including any decisions made on announcement, such as whether it was kicked or its Buyback cost was paid. Effects on the spell, such as Sleight of Mind or Flashback, are not copied.

If the copied spell requires a target and there are no legal targets, then the copy has the same target (which should be illegal) as the original.

If the spell is countered before the triggered ability resolves, then the ability will make a copy of how the spell looked right before it was countered.

It triggers once per spell, so you can’t get multiple copies of a spell using this card. And since this card is legendary, you can’t control more than one.

The copy is not “cast” so it will not trigger anything that triggers on a spell being cast.

You choose whether or not to pay when the triggered ability resolves.

You are not required to choose new targets. If you do choose, then the targets must be legal. If you don’t change them, the spell is placed on the stack whether or not the targets are legal. The legality of the targets is always checked on resolution of the spell, so the spell may be countered at that time if all of its targets are illegal.

Mirari : règles, fonctionnement et interactions (MTG)

Mirari is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you cast an instant or sorcery spell, you may pay . If you do, copy that spell. You may choose new targets for the copy.

How it works : Mirari

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
Everything about the original spell is copied, including any decisions made on announcement, such as whether it was kicked or its Buyback cost was paid. Effects on the spell, such as Sleight of Mind or Flashback, are not copied.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
If the copied spell requires a target and there are no legal targets, then the copy has the same target (which should be illegal) as the original.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
If the spell is countered before the triggered ability resolves, then the ability will make a copy of how the spell looked right before it was countered.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
It triggers once per spell, so you can’t get multiple copies of a spell using this card. And since this card is legendary, you can’t control more than one.
No other card is named in this ruling.
#5October 4, 2004· Wizards of the Coast
The copy is not “cast” so it will not trigger anything that triggers on a spell being cast.
No other card is named in this ruling.
#6October 4, 2004· Wizards of the Coast
You choose whether or not to pay when the triggered ability resolves.
No other card is named in this ruling.
#7October 4, 2004· Wizards of the Coast
You are not required to choose new targets. If you do choose, then the targets must be legal. If you don’t change them, the spell is placed on the stack whether or not the targets are legal. The legality of the targets is always checked on resolution of the spell, so the spell may be countered at that time if all of its targets are illegal.
No other card is named in this ruling.