The Power That Preserves is the third book in the Thomas Covenant series. Thomas Covenant is a leper in the real world who is called back to the Land, a place that Covenant is certain is only a figment of his imagination. Although he has no belief in the Land he has grown to love it and undertakes the journey to destroy Lord Foul, the destroyer of the Land. Covenant is able to save the Land but before being transported back to his own world he is given the choice of living out his life free of leprosy in the Land or returning to his world. Covenant chooses his own world and is happy to be alive.

Covenant is back in his reality from another trip to the Land where his daughter, Elena, dies. Covenant blames himself for this death and is having a hard time coming to grips with reality. He eventually winds up taking a walk in the woods where he goes to sleep. When he awakens he finds a young girl cornered by a rattlesnake. Covenant gets up to help the girl but falls and hits his head before he gets to her and is summoned to the Land.

Mhoram and the others in Revelstone have called Covenant but he begs them to let him return to his home. Mhoram explains before he releases Covenant that the ancient servants of the Lords, the Bloodguard, have been corrupted by the evil Lord Foul and the Land is dying under Foul's attack. Covenant returns to the girl and she has been bitten already. Covenant lances the wound and sucks much of the poison out and begins carrying her back to her parents. Once there Covenant sits down and is pulled back to the Land. Covenant realizes he has not been summoned by Mhoram but by Triock and Foamfollower, people he knows from previous journeys to the Land. Foamfollower and Covenant start their journey to Lord Foul where Covenant hopes his magic ring will be able to help him defeat Lord Foul. Meanwhile Revelstone is under attack by Lord Foul's army.

When Foamfollower and Covenant reach Lord Foul, Covenant is able to realize how to defeat Lord Foul and has Foamfollower laugh. The laughter triggers an emotion that helps defeat Lord Foul. Covenant uses his magic ring to destroy Foul's evil illearth stone. As thanks for saving his creation, the Creator offers to let Covenant live out his days in the Land with health. Covenant chooses to return back to his world where he lies dying from his leprosy and the anti-venom given to him after he tried to help the girl. The Creator gives Covenant's body the strength to survive and Covenant is happy to be back home and alive.