The story centers upon Ankh-Morpork City Watch Commander Sam Vimes and his colleagues, including Captain Carrot, as they solve the apparent murder of a dwarf that leads to tensions between the city's trolls and dwarfs, two groups often at odds. To prevent all out war between the two groups in the city and elsewhere on the Discworld, Vimes and his family travel to Koom Valley, where a battle occured that both sides claim was the beginning of the other's attrocities against them.
This book deals with race relations and drug use, but has an uplighting ending. I recommend it highly, along with the other 38 Discworld books Pratchett wrote. As the seventh novel about the City Watch, "Thud!" makes more sense if one reads the other six first, starting with "Guards! Guards!"