The plan was to have Fandral turn Stephen in to the Port Royal authorities to collect the bounty. Stephen would then escape using his magic long before he was introduced to the hangman, but something happened to disrupt the perfect plan.
"There's always gotta be a twist," he muttered sarcastically under his breath.
When he was arrested and thrown into the prison cell, unbeknownst to him, somebody took the opportunity to pick Stephen's pocket and walk off with his Sling Ring. The thief wouldn't know it was magic, much less know how to use it, and probably thought it was some curious bit of jewelry. Stephen could still easily escape without it, using any variety of spells at his fingertips, but he wanted his ring back. If the week ended, and they returned to campus without it, Stephen understood that he wouldn't be coming back with him. It was far too important an artifact to just leave behind, but Stephen was also attached. That ring was with him from the beginning of his apprenticeship as a sorcerer, and thus held sentimental value. Besides, people like Natasha and Loki would never let him live it down that he'd lost it.
"Okay. Where to begin?"
Cloak, which had transformed into a red pirate coat for the week, gave a tug on the shoulder to get Stephen's attention. He turned his head in that direction, now hearing the echo of footsteps coming down the stairs, more than one set. The jailers, Stephen assumed, and he squared off to face them through the bars of his cell. He wasn't scheduled to hang for a few more days now, and it wasn't time for his meal, so why were they coming? In any case, he could use the chance to check to see if these guards had his ring or not... it was a start.