Martian Divides picks up the Martian Symbiont story two years after the end of Martian Blues. The Martians who returned to Earth hundreds of years ago finally found human women who were compatible with them and their telepathic symbiont. With the additional issue of defending Earth from hostile aliens now resolved, the Martians decide it's safe to return to their one true passion: space travel. Only one group of beings can threaten their plans. Angry that most other people now carry the alien telepathic life form, the underground insists that what's left of real humanity is in danger. They believe Martian influence on society puts them at a disadvantage, so they set out to prove their humanity still has a voice. When they attack a Martian, the resulting mental shockwave has consequences felt around the world.
The Martian matriarch chooses not to delay the planned return to outer space. However, faced with the prospect of losing her telepathic leadership, humans with the symbiont insist on a successor. To their dismay the underground comes to realize they might not be getting rid of all the aliens. Loyalties and friendships are tested as events unfold along various sides of the Martian divides.