The Truth Serum was discovered a long time ago. A doctor in the United States found that after patients were anesthetized by a certain drug, they would unconsciously answer many questions accurately.
The human mental state can be divided into three levels: conscious awareness, subconsciousness, and unconsciousness. When the Truth Serum is used, the person under interrogation loses their conscious awareness. At this moment, if you ask them questions, they will reveal everything they know from their subconscious.
Of course, the Truth Serum isn't always effective, nor is it omnipotent. For instance, it doesn't work on mentally ill individuals because they perceive their hallucinations as a real world. Using the serum on them would yield nothing but incoherent ramblings of a madman.