Zethi's last ability triggers, causing you to pay ({3} magic symbol) ({W} magic symbol) to cast a copy of Cosmic Intervention, creating a replacement effect that is active this turn.
Move to your postcombat phase.
Cast Walk the Aeons with buyback by paying ({4} magic symbol) ({U} magic symbol) ({U} magic symbol) and sacrificing three Islands, putting the Islands in exile instead of the graveyard and creating a delayed triggered ability due to Cosmic Intervention's replacement effect.
Resolve Walk the Aeons, causing you to take an extra turn after this one and returning Walk the Aeons to your hand.
At the beginning of your end step, Cosmic Intervention's delayed triggered ability triggers, returning the exiled Islands from exile to the battlefield.