Midway through the episode, Crane smacks his head and is absent for the rest of the show (in sickbay with a serious concussion). He also misses most of an upcoming episode ("Fatal Cargo"). It is possible that David Hedison needed some time off to film an appearance on the variety show The Hollywood Palace, where Hedison performed in a spoof of Voyage with host Milton Berle. At the end of the episode, after Crane is noted as being "Seriously Injured", he is shown in the control room chatting about the adventure with not a single mark on him. He must have a mutant healing factor.