The story is set after the conclusion of Trace 1 and centers on Han Sihyun, an investigator with the National Investigation Service (NIS). In an attempt to find a way to coexist with "Troubles" rather than killing them, Sihyun frequently proposes to the NIS board that they approve research into communicating with "Troubles." However, after his proposal is rejected once again, Sihyun accidentally captures a "Trouble" while drunk and decides to conduct illegal experiments on it. Although communication seems impossible at first, Sihyun and his "Trouble" gradually begin to close the distance between them and form a bond.