As the group infiltrates Monroe's compound through Philadelphia's old subway tunnels a cave in blocks their way out. With oxygen levels running low everyone begins hallucinating their fears.
Meanwhile, Rachel is almost successful in convincing Monroe the amplifier she's building is almost done, but Tom begins to see that she's lying. Monroe brings in a second expert to confirm or deny what Rachel is telling them. It turns out Rachel is building a bomb. Some quick thinking by Rachel buys her some time as she kills Monroe's second expert, allowing her to be the only person with the expertise he needs to truly have an amplifier.