 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 . 
