Riku's first ability triggers, causing you to pay ({U} magic symbol) ({R} magic symbol) 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
Temporal Manipulation, and returning the original
Narset's Reversal to your hand.
Resolve the copy of
Narset's Reversal created in step 5, copying
Temporal Manipulation and returning the original
Temporal Manipulation to your hand.
Resolve
Temporal Manipulation, causing you to take an extra turn after this one.
Repeat each turn.
Notes
You may pay an additional ({U} magic symbol) ({R} magic symbol) to copy the extra turn spell, and only need to do this loop during your second extra turn.