"Are you sure about this?" Potter's voice asked, sounding unconvinced about something.
Black snorted arrogantly and slapped his friend on the back. "When have I not been sure of any prank I've ever pulled, Prongs? This will be the most talked-about feat ever to be accomplished within the walls of Hogwarts. We'll be legends!"
"Mm. But will we still be students here? Not to crush your hopes of fame, but what good is becoming a legend if it means we're expelled?"
"We won't be expelled! I've got the entire plan worked out; no one will even know it was us that pulled it off." Slightly lowering his voice, he added, "I can always pin the blame of Snivellus, if the need arises."
Severus had to dig his fingernails deep into the spine of the book he was holding in order to prevent himself from hexing the the two idiot Gryffindors until they were a lovely shade of Slytherin green. He silently extinguished the light of his wand, and waited breathlessly as the voices and footsteps came closer.
"Madame Pince is out there," Black hissed at Potter. "Where's your invisibility cloak?"
"Right here, just a second."
There was a sound of fabric rustling, then quick footsteps, then silence. After Severus was completely sure that they had left, he put the book he had been browsing through back into its rightful place on the shelf, and left the restricted section as inconspicuously as he could manage.
Whatever it was that Black and Potter were planning, it was going to be nasty. Why else would they have been in the restricted section? The thought to expose their plot to the headmaster or another member of the staff occurred to Severus, but he pushed it away for the moment. If he told someone now, all Black and Potter would have to do would be to deny the claim. Instead, Severus made up his mind to wait until the plan the two Gryffindors had created was about to be set into motion, so their involvement could not be questioned when they were discovered.
Severus hoped they would be expelled.