Riku of Two Reflections + Narset's Reversal + Temporal Mastery

Steps

1. Cast Temporal Mastery by paying {5}{U}{U}.
2. Riku's first ability triggers - choose not to pay.
3. Holding priority, cast Narset's Reversal by paying {U}{U}, targeting Temporal Mastery.
4. 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.
5. Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Temporal Mastery, and returning the original Narset's Reversal to your hand.
6. Resolve the copy of Narset's Reversal created in step 5, copying Temporal Mastery and returning the original Temporal Mastery to your hand.
7. Resolve Temporal Mastery, causing you to take an extra turn after this one.
8. Repeat each turn.