Requires

Spelljack must be in hand. Swarm Intelligence must be on the battlefield. You have an additional instant card in hand. Mana needed: plus enough mana to cast the additional instant card

Steps

1. Cast any other instant card from your hand by paying its mana cost.
2. Swarm Intelligence triggers, choose not to create a copy of the instant.
3. Holding priority, cast Spelljack by paying targeting the instant remaining on the stack.
4. Swarm Intelligence triggers, creating a copy of Spelljack that is targeting the original Spelljack.
5. Resolve the copy of Spelljack, countering the original Spelljack and exiling it with the ability to cast it for free from exile.
6. Cast Spelljack from exile without paying its mana cost, targeting the instant cast in step 1.
7. Repeat from step 4.

Results