Who Unstriped the Zebra?

The Zebra is a shy and discreet animal. He wakes up one morning with a plaid pattern on his flank. Someone has wanted to take revenge on him for winning the contest of best striped animal in the zoo. Who did it? The Okapi? The Snake? The Tiger? In the meantime, Hermione has the blues, she's convinced that she'll never be a good detective. Sherlock has to cheer her up.