From the introduction:
Three years after his defeat in the Fourth Holy Grail War (depicted in Fate/Zero), Waver Velvet returned to the Mage’s Association’s Clock Tower in London, to resume his studies in magecraft. He became employed there as a lecturer for third year students, and quickly became popular among the mages who lacked individual influence and long family histories, eventually revolutionizing thaumaturgic education. He rescued his dead teacher’s house from ruin by compiling the man’s leftover notes and research, and in doing so earned the title “Lord El-Melloi II,” the man who revived the house Archibald. His mediocrity as a magus prevented him from advancing in practical skill, but in time he became one of the greatest professors in the Clocktower.
Now in only the mages around him could stop murdering each other and getting him involved.