21D—of itself, it signified an aisle seat in the 21st row of the Boeing aircraft. To Mr. Harrison, librarian, the letter signified the World History section of the Library of Congress catalog, while the number brought to mind the miserable century in which he now found himself—the century that had birthed the alien seated next to him.

Not that there was anything revolting or unusual about the teenage miss huddled against the window in 21E. But he knew that the glowing screen in her hands possessed an artificial intelligence that was relaying messages to her brain by slender wires connecting to the clean white pods nestled in her ears. This frightened him. In the 15 minutes since he sat down, she had not looked at him more than once nor spoken to him at all. He found his attention riveted by the contrast between the dull impassivity of her features and the exciting musical strains—inaudible to him—which must be stimulating her skull.

The flight attendant paused at row 21. Would she please turn off her cellphone for takeoff? The girl disconnected, and the screen went dark.

Mr. Harrison now greeted her and, lo, she replied! He followed this with some comment of quirk an old librarian might make, and she half smiled. As the plane rumbled down the runway and diagonalled into the sturdy air that cradled it, she watched the neighborhoods fly by, admiring. Mr. Harrison thought of human history, its tremendous cache of knowledge enriching mankind, he its custodian. Where was it tending? Into death and darkness? Into the clouds?

When the aircraft completed its initial climb, the girl’s wonder waned. At 10,000 feet, she replaced the pods into pink ears, reconnecting to the music. Her eyelids lowered by half, her expression slackened. In the past, young people danced to music, thought Mr. Harrison, taking up his book. Now they drooled over it.

For three hours, the plane marched forward, the situation in row 21 unchanged: the girl absorbing, the librarian reading and thinking.

Mr. Harrison noticed that the teenager took no thought for herself; as a knight of customer service, he came to her aid. He asked whether she would like a complimentary pillow to fill that space between her head and the hard window; she said yes. When the flight attendant proffered pretzels, he knocked politely on her shoulder to solicit a response. And when, eyes fixed forward on her tray table, she monotoned, “Gotta use the bathroom,” he patiently attempted to clarify whether she wished him to stand up and let her out. She did wish it, and he obliged.

After three hours, the plane descended from the clouds. The girl again complied with the ban on electronic signals, and her eyelids returned to full mast.

“Nice view,” she said, her first words since the bathroom comment.

“Indeed,” he replied, much pleased at this baby step in human communication, and wondering if Anne Sullivan had felt like this. Another strange joke brought the other half of her smile, completing his impression of a young lady who had to try hard to stave off happiness. But she knew how to go about it: the plane landed, and she plugged back in.

As she exited the plane, he saw the wires running down her cheeks, obscuring her pretty face. Twice only had he seen her unplugged, natural—at takeoff and at landing. He sighed, and his final reflection was this. We are born in majestic silence, and at death we depart with reverent quiet. But in the short gap of life, we sedate ourselves with “let the music play.”