Days of Grass is subtitled "After the fall of humanity", and it is set some one hundred and forty years after the invasion of earth by aliens, nicknamed 'spiders' for their eight-legged vehicles. The scattered remains of the human race exist in small groups mainly underground in shelters.

Esther is a slim dark haired girl living in one such shelter. Going above is forbidden and few realise why, the aliens are a secret. Esther stumbles upon a pipe leading to the surface, at first frightened, she is drawn back repeatedly, roaming the deserted city above.

Tanith Lee manages to create a twilight world underground, redolent with dark tunnels and the inevitability of known paths. The people are increasingly ignorant and petty, squabbling amongst themselves, vying for scraps of power in a world where there are only limited places and no real benefits. Soldiers endlessly drilling for no real purpose, no goal.

Eventually, inevitably, Esther is caught, given away by her tanned skin in the dim tunnels underground, she is taken to the leader, Standish. Standish is an old man, living in his own suite of rooms with treasures from above, including books. Esther is given the freedom to roam the apartments and read, and use the trap door leading from them to the surface.

Standish appears to be testing her, trying to find a leader to replace him. For the last few generations the leader has been a hereditary title, and he has no children, although he suspects Esther may be his bastard child. She is disinterested, despising the other inhabitants of the tunnels as weak and directionless.

As she grows older, she discovers that the soldiers and other inhabitants believe she is the lover of Standish, and that she is getting preferential treatment from them because of this. The power gained from this begins to interest her.

One day while wandering through the city, she finds a human, a man, called Cury. He begs to come down with her, describing living hunted, alone, frightened. She allows him to follow her back, but then discovers that Standish has finally died, forcing her to decision about the leadership. She manipulates the soldiers into accepting her and Cury is allowed amongst the people.

While she hates the people she now leads, she wants the pow, but that is all destroyed by a catastrophe. There is a fire, the generator is destroyed, and a way is opened to the outside world.

Outside, the spiders wait.

Cury was a spy, raised by the aliens as a pet, one of their dogs, hunting out the hiding humans. The group are marched to a seaside city, placed in a compound, except for Esther. She belongs to Cury. He wants her for his pleasure, but not for sex, he wants her to abuse him, which she does, disinterestedly. She is allowed a man to visit her, one of the soldiers, Steiner, who becomes her lover.

Her hate and misery are pointless, useless, she cannot kill herself yet she can see there is no point fighting. The Cury takes her to meet one of the aliens, his Gods. He calls the alien Jupiter, having made up names from the pantheon of gods for all the aliens he knows.

Esther is disgusted to learn that Jupiter is humanoid. He can read her mind, is aware of her every impulse and emotion, and there is no escape even in her own mind. She is given access to the museum of human relics, books, and Cury no longer visits her.

In many ways this can be read as an indictment of colonialism, and Tanith Lee certainly would have been aware of that aspect, the comparison is a strong one. The humans are bought off with a placid life and security, no longer trying to rebel, and they become largely irrelevant to the main action, between Esther and her enemies.

After some time she is moved to a new apartment, obviously designed for one of the aliens, obviously for Jupiter's pleasure. She anticipates his plans, that he will want her. She taunts him, referring to bestiality and his reputed superiority.

After he has slept with her, and her hatred and disgust are at their peak, he tells her the truth. He has destroyed his species. He is the only one left, the rest are dead and the earth is free, but Esther must lead the humans in the time to come. She must destroy him.