You have an additional instant or sorcery card in hand.
({3} magic symbol) ({U} magic symbol) plus enough mana to cast the additional instant or sorcery available.
Steps
Cast the additional instant or sorcery by paying its mana cost, triggering Eye of the Storm.
Holding priority, cast Spellshift by paying ({3} magic symbol) ({U} magic symbol) targeting the instant or sorcery spell.
Eye of the Storm triggers, exiling Spellshift and casting copies of Spellshift and any other instants and/or sorceries exiled by Eye of the Storm of your choosing without paying their mana costs.
Choose to have Spellshift target any instant or sorcery spell on the stack of your choosing.
Resolve Spellshift, countering the chosen instant or sorcery spell and then revealing an instant or sorcery card from your library, cast it without paying its mana cost, and shuffle your library.
Eye of the Storm triggers, exiling the instant or sorcery and casting copies of Spellshift and any other instants and/or sorceries exiled by Eye of the Storm of your choosing without paying their mana costs.
Repeat from step 4.
Results
Near-infinite casts of all instant and sorcery spells in your library and exiled by Eye of the Storm.