A smiley face on the pavement

The weekdays of the St. Petersburg tour guide Artem Zubarev intersect with murders twice within a couple of days: first, a dead girl is found in the Alexander Garden, who had turned to him for help the day before, and then a woman is thrown under a river tram right before his eyes. Coincidence? Bad luck? Either way, Artem is involuntarily drawn into a series of strange events, and the retired investigator Bragin has a precise and concise definition for this series: a series. There are actually many more murders, and the connecting link between them is a smiley face drawn near the crime scene. A similar series of mysterious deaths had already occurred in St. Petersburg before the revolution - at least that's what is said in a diary of a Russian emigrant that Artem accidentally came across. Perhaps this diary will be the key to solving the current murders and deciphering the meaning of the smiley faces drawn on the asphalt?