The Professor and Mr. Burke are out dining with Ginny, and another patron tries to take her chair and points out that there's no plate and no-one sitting there. The Professor tells Burke that he is finished with his equations and they can "begin testing on controlled fission of non radioactive materials." While driving, the Professor keeps muttering about murdering fools and then starts driving erratically at a high speed.
Burke consults with Dr. Cottrell who wonders why the Professor should want to kill himself. He looks in on the Prof who is talking to his invisible dead daughter, saying "we'll all be with you soon." Burke relates that after taking the wheel, he told the Professor that he could have killed the both of them. The doctor slams the door and exclaims that the Professor knows his daughter is dead, that he wants to commit suicide but cannot consciously do so. He shares that the Professor has worked out the means to create fission using non radioactive materials and that when the world goes up in flames, he will be with his daughter again and avenge the murderer of his little girl.
Once again, Rod Serling clearly demonstrates the idiocy of the nuclear arms race. While the ending is still frightening today, it was likely even more so during the Cold War when kids were practicing Duck and Cover at school, families were building bomb shelters, and the detonation of a nuclear weapon seemed inevitable. Chilling indeed.