In Chapter 4 of Book Three, where does Finch go and why is he there? What thought “frees” him?

Finch is actually on an acid-trip. He wants to understand V's psychology. Finch sees a building close by with lights on and people inside. As he hallucinates, he sees Dr. Surridge before him. Although he explains that there are things bothering him, she brushes him off on Father Lilliman. In this sequence, she calls to Lewis Prothero to take him away. As Finch is dragged off, he sees Delia kneeling to the priest and kissing his ring. As Finch's hallucination continues, he is tossed into a cell, which he quickly realizes is Room Five, where V was imprisoned. He mixes the chemicals that are on the floor. He now realizes that he is keeping himself a prisoner there. Once he sees this, he knows that he must break free. He sets off the explosives that the chemicals create, and he leaves the building a free man. This makes him deliriously happy.