Sammy is spending her school detention time helping out at St Mary's church - boring, but peaceful, she reckons. Unfortunately not. A trio of singing nuns arrive in a motorhome, and prepare to turn the whole place upside down! Plus Father Mayhew's cross has disappeared, and his main suspect is the bad apple who's working out her detention - none other than Sammy Keyes. It's up to Sammy to find the real thief, before Father Mayhew tells the police...