When the diamond on Miss Mackle's engagement ring disappears, the kids in Room 3B are determined to help her find it - especially Harry. The third grade talent show is coming up, and he wants to show off his skills as South School's greatest detective. Meanwhile, Doug has no idea what to do for his act. He can't sing like Dexter, or dance like Ida, or solve mysteries like Harry. Now the best friends have to find both Doug's talent and the teacher's diamond, or else the talent show will be a horrible disaster for everyone!