Activate Jace's zero loyalty ability, putting Karn's Temporal Sundering on top of your library.
Pass the turn.
On your next draw step, when you draw Karn's Temporal Sundering, reveal it due to Kefnet's ability, making a copy that costs ({2} magic symbol) less to cast.
Cast the copy for ({2} magic symbol) ({U} magic symbol) ({U} magic symbol) , granting you an extra turn and returning up to one nonland permanent to its owner's hand.