Back at the lab, Astrid's finished the analysis on the blood sample he gave her on the moth. Turns out it had a genetic disorder: "Ah, germline mutation. Just as I thought. This man has the exact same disorder," he says. Astrid asks if that's what he thinks lets them transform. "No, the mutation is what makes them deformed. I have no idea what makes them transform," he says. But he's got the strange feeling he's seen this mutation before. And then he starts humming Carmen, and Astrid starts whining about that, but of course now it triggers the memory that he worked on Project Elephant because OF COURSE HE DID, and then he puzzles out the song's meaning. Well, Astrid does -- after writing the words down she asks if "Harkness" means anything to him, and he says it's a library at the law school and then clues in that the song is a mnemonic and they haul ass to find out what's at the library.
Over in Edina, Olivia and Peter are coming up with sweet nothing as the entire F-section in the tax records is gone, suggesting that someone stole it. Olivia decides to check the federal records: "If you lived here, you must be on the census."
Over at the Harkness Law Library, it's all started to come back to Walter, which is fortunate, because I don't see how he would otherwise have remembered to unscrew a metal grate near the bottom of a bookshelf. While he's doing that, Astrid asks him what happened in Edina; he seems like a different person since he got back. Fits in with the theme! Walter says he's always found work to be restorative: "When Peter was sick, it was the only thing that kept me going. Amazing how it heals the soul," he says.
He's finally unscrewed the grate, the purpose of which in this part of the bookcase is beyond me (and considering how old this bookcase looks, it's lucky there haven't been any renovations done), and he pulls out a box of Devil Dogs, and he gets all excited, and you'd be forgiven for thinking that it's just a box to hold whatever Walter has stashed, but no, it's an old package of actual Devil Dogs that Astrid takes from him, promising him that they can get a fresh package. There's another box in the bookcase, this one containing files and pictures -- of people with grotesque facial deformities, just like the man in the lab, Astrid unnecessarily explains for us. "I understand now," says Walter.