Enterprise is chosen as the testbed for the revolutionary M-5 computer. The machine's inventor, Richard Daystrom, claims it can take control of a starship, freeing men for less dangerous and more rewarding work. It all depends on whether his breakthrough technology, multitronics, is as powerful and capable as he says. After a few exercises, Spock begins to wonder exactly how multitronics works - for the M-5 does not react as computers typically do.

The Enterprise has used the "light speed breakaway" technique to travel to Earth in the year 1968, to study how humans survived the desperate problems of that era. Then, they intercept a man beaming back to Earth from at least a thousand light years away, a technical feat beyond even Federation science! Who is Gary Seven, how did he get access to such advanced technology, and why has he come to Earth? He has a story, but Kirk has trouble believing it. Worse, he has trouble knowing whether he should allow Seven to pursue his agenda, or try to stop him. One of those choices is certain to destroy the future... but which?