Cast Twice Upon a Time by paying {4}{U}{U}.
Riku's first ability triggers - choose not to pay.
Holding priority, cast Narset's Reversal by paying {U}{U}, targeting Twice Upon a Time.
Riku's first ability triggers, causing you to pay {U}{R} to copy Narset's Reversal, with the copy targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Twice Upon a Time, and returning the original Narset's Reversal to your hand.
Resolve the copy of Narset's Reversal created in step 5, copying Twice Upon a Time and returning the original Twice Upon a Time to your hand.
Resolve Twice Upon a Time, causing you to take an extra turn after this one.
Repeat each turn.

Infinite turns

Riku of Two Reflections

Narset's Reversal

Twice Upon a Time // Unlikely Meeting

Deal combat damage to an opponent using Efreet Flamepainter.
Efreet Flamepainter triggers, causing you to cast Unlikely Meeting from your graveyard without paying its mana cost.
Resolve Unlikely Meeting, causing you to take an extra turn after this one and exile it.
Activate Isochron Scepter by paying {2} and tapping it, casting a copy of Pull from Eternity without paying its mana cost.
Resolve Pull from Eternity, putting Unlikely Meeting from exile into your graveyard.
Repeat each turn.

Combos with "Twice Upon a Time // Unlikely Meeting"