Andrés looked at his own solution for 7.12. He had forgotten the sign convention for mutual inductance. One minus sign. That was all. He corrected it, and the infinite current vanished, replaced by a beautiful, decaying oscillation.

That’s when his lab partner, Elena, slid a note under his door.

The ghost has the key. Aula 3.12 was a forgotten lecture hall on the basement level, where the hum of the ventilation system sounded like a dying capacitor. At 11:00 PM, Andrés found three other desperate souls waiting: Elena, a quiet transfer student named Farid, and a pale, intense girl everyone called "La Ingeniera" because she had already finished two internships at Iberdrola.

It was not a manual for copying. It was a manual for understanding . The ghost—whoever wrote it—had been a brilliant, compassionate teacher.

He laughed out loud. The others looked up, bleary-eyed.