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This is the last episode of TOS in which Enterprise crew members (Wyatt, D'Amato, and Watkins) die onscreen or close to it. However, in Requiem for Methuselah (1969), Kirk will report in his opening log that three crew members have died of Rigellian fever.
Lt. Radha is both the first explicitly Hindu character (shown by the bright red dot on her forehead, known as a Bindi), and the first Enterprise helmswoman, to appear in Star Trek.
The center section of D'Amato's tricorder differs substantially from the standard Starfleet model. Instead of tape discs and a moiré pattern, it features an intermittently glowing white panel and what appears to be a tubular sensor. In deference to D'Amato's specialty, some prop-conscious fans have dubbed this a "geological tricorder."
Second and final appearance of Dr. M'Benga.
A new access tube was created for this episode to show where the matter-antimatter reaction chamber was.
In this episode it is revealed there are (at least) three Doctors assigned to the USS Enterprise: Chief Medical Office Dr. Leonard McCoy, Dr M'Benga (also featured in A Private Little War (1968)), and Doctor Sanchez (only appearance).