Deela was the leader of the Scalosians. An accident had left her people accelerated into a time frame where they moved far faster than the rest of the universe. The accident had also left all their males sterile, with only a handful of their people surviving. To lure males to Scalos for breeding purposes, Deela sent out a distress call which was picked up by the Enterprise. A landing party visited the surface and Ensign Compton was accelerated to the Scalosians' time frame after drinking their water, being paired with a Scalosian female.
Deela and a number of her people accompanied the rest of the landing party back to the Enterprise via their transporter beam. Once there, they began setting up a machine designed to place the entire crew in suspended animation so they could be retrieved whenever the Scalosians needed new mates, protecting it with a forcefield to stop the crew from intervening. Deela then placed Scalosian water in Captain Kirk's coffee, having chosen him as her mate, resulting in him being transferred to their time frame. The transfer meant their accelerated bodies would die from even a minor injury and Deela reluctantly let Kirk that believe that he, rather than her aide Rael, had caused Compton's death. Deela's accelerated state meant she could simply dodge phaser blasts.
Despite having feelings for Rael, Deela also grew fond of Kirk and they became lovers, with her protecting him from Rael's jealousy. She allowed Kirk to record a warning message to his crew about the Scalosians' plan, believing it would be complete before anyone could hear or act on it, and covered up an act of sabotage on his behalf, claiming it was a malfunction. She was disappointed when Kirk apparently became docile and compliant with their plan, admitting she preferred his past spirit.
Deela prepared to beam down to Scalos with Kirk but he dropped his pretense and gave her the slip. Teaming up with Commander Spock, who had deliberately transferred to the Scalosians' time frame, he stunned Rael and destroyed the Scalosians' machine. Deela surrendered without further resistance, allowing Kirk and Spock to beam her and Rael down to the planet's surface. Kirk confirmed the Federation would quarantine the area to prevent any more ships being lured there, meaning the Scalosians were likely doomed to die out.
