Izzet Chemister and Helm of the Host on the battlefield.
Helm of the Host attached to Izzet Chemister.
({1} magic symbol) ({U} magic symbol) ({R} magic symbol) available the first turn, with ({1} magic symbol) ({R} magic symbol) ({R} magic symbol) each extra turn available.
Steps
Cast Time Walk by paying ({3} magic symbol) ({U} magic symbol) ({U} magic symbol) , causing you to take an extra turn after this one.
At the beginning of your combat phase, Helm of the Host triggers, creating a token copy of Izzet Chemister.
Activate the Izzet Chemister token's first ability by paying ({R} magic symbol) and tapping it, exiling Time Walk from your graveyard.
On your next turn, activate the Izzet Chemister token's last ability by paying ({1} magic symbol) ({R} magic symbol) , tapping it, and sacrificing it, casting the exiled spell without paying its mana cost.
Resolve that spell, causing you to take an extra turn after this one.