Guardian is set on the Earth-like planet of Haixing, populated by a native human population and two alien races — the Yashou, who are able to shapeshift into plants and animals, and the Dixing, superpowered beings who live deep underground. The three races have enjoyed a peaceful co-existence since the arrival of the Yashou and Dixing over ten thousand years ago, broken only by an ancient war declared by the Dixing after a meteor strike depleted their resources. During that war, the Haixing and Yashou were led by the hero Kunlun, who became a close comrade to the Black Robe Envoy, the leader of a faction within the Dixing that sought peace. The war was won by the Haixing with the use of four holy tools crafted from the meteor. At the end of the war, Kunlun vanished, and the Black Robe Envoy became the overseer of peace between the races while awaiting Kunlun's return. In the present, conflict has begun to erupt again between the Dixing and the surface-dwelling races. The hotheaded Zhao Yunlan leads the Special Investigation Department (SID), a group that works with the Black Robe Envoy to uphold peace and investigate cases related to the Dixing people. While investigating an incident at the local university, Zhao is assisted by Shen Wei, a biological engineering professor. The two men are drawn closer together as a mysterious enemy threatens to re-ignite war