A mysterious mobile phone application called "Echo" is quietly spreading in Donghai City. It cannot be uninstalled, and will issue strange tasks to the selected "players" at midnight: find a non-existent ward in an abandoned hospital, send a letter to a non-existent passenger on the subway at midnight... You can get rewards for completing the tasks, but the price of failure is to be completely "erased" by an incomprehensible weird force.
Chen Mo, a city memory restorer, was accidentally involved in this deadly game during an exploration work. With his astonishing familiarity with the corners of the city and meticulous logic, he has repeatedly cracked the rules behind the mission on the verge of life and death. In order to protect himself and find out the truth, he met Lin Wan, an independent journalist who was also investigating the "disappearance" incident. With a calm analysis and decisive action, the two joined forces to fight against the bottomless malice behind "Echoes". They gradually discover that this app seems to be awakening ancient fears sleeping under the urban fabric, and what they have to face is a huge abyss that is far beyond imagination and is eroding the real world.
