Elizabeth debates the ethics of leashing Six-Thirty, believing the dog’s judgment surpasses most people’s. A new city leash law and her growing attachment push her to buy one, despite Calvin’s confidence that Six-Thirty needs no tether. Rowing is canceled for lightning, and Dr. Mason floats a spring regatta, which both Elizabeth and Calvin pretend to welcome.

Two days later, before dawn, Elizabeth cannot sleep and leaves for the lab early, setting potatoes in the oven and a note for Calvin. Calvin wakes alone and decides to run to work with Six-Thirty so they can all ride home together, driven by his worry about Elizabeth traveling alone.

On the way out, Calvin sifts through opportunistic letters, remembers a lost pen-pal friendship that ended when he admitted he hoped his father was dead, and resolves to keep his living father a secret from Elizabeth. He rationalizes that some secrecy is normal, reflects on how love has changed him, and feels grateful for Elizabeth, Six-Thirty, their work, and rowing.

At 5:18 a.m., picturing Elizabeth at her centrifuges, Calvin prepares to run the five miles to Hastings. In the dark drizzle, Six-Thirty hesitates; Calvin remembers the new leash, clips it to the dog for the first time, locks the door, and leaves. “He was dead thirty-seven minutes later.”