Because Edward Hardwicke was filming elsewhere, Mycroft (Charles Gray) assists Holmes in this case.

Holmes rose, dusted his knees, and adjusted his cravat. He looked at Eleanor, then at me. The languid, theatrical mask was back in place, but beneath it, I saw the steel.

At the forty-sixth minute, he sat back on his heels. He looked up at me, and for a fleeting second, the mask of cold logic slipped. There was a flash of raw, almost childlike relief in his eyes. “It is done, Watson. The mechanism is frozen.”

“No,” he said, quietly, to the door. “But he is about to be.”