 The episode followed the theft of cryogenically-frozen heads by shapeshifters from the parallel universe in their search for a specific head, while the Fringe team attempts to prevent this. Meanwhile, Olivia struggles to remember her conversation with William Bell (guest actor Leonard Nimoy) that took place in the season one finale. "Momentum Deferred" marked the first appearances of guest actors Theresa Russell, Sebastian Roché, and Ryan McDonald.
