On St Nicolas' Eve, jewellery and wristwatches worth fifty thousand crowns are stolen from a jewellery store. The camera installed in the store captured the culprit in the traditional mask of the St Nicolas' Eve devil and the photograph suggests that the thief is a child. Doctor Martin, however, has already managed to solve the mystery in another way. The jeweller joined with a seller of devil's masks and they committed the theft together. To get an alibi, they moved the time on the watch nine minutes back. Martin recognizes the trick when he gets hold of a wristwatch that little Martina took by unfortunate accident.