Three energy beings from outer space have come to observe Earth. Each of the three is actually a composite of three individuals, who have joined together in a bond.
Arriving, they find that humans are very promising--and, unexpectedly, very violent. Investigating, they find that humans are all infected with an energy virus--"Pa'ak, the most vicious, most contagious energy-virus known." This virus causes discord, and humans have it particularly bad:
They resolve to split up to investigate this plague, in hopes of curing it. However, they are particularly vulnerable to the Pa'ak. In order to protect themselves while they investigate, they separate into nine individuals--much weaker, but able to enter human hosts and (somehow) be protected from the Pa'ak.
There is a problem, however. In order for the energy beings to recombine, they must inhabit humans who have bonded emotionally--a threesome of such humans--and the foolish humans just won't do it!
If they can't rejoin, they won't be strong enough to do anything about the Pa'ak, and they must not allow the humans spread the virus throughout the galaxy. There may be no other recourse than to goad the humans into atomic war--into destroying themselves, so that they will not be able to leave the planet.
Eventually, however, they manage to gather a group of people in a satisfactory configuration to enable them to rejoin, and they begin the work of saving humanity from the Pa'ak.