Mark Slate is an enforcement agent for the United Network Command for Law and Enforcement. In his initial incarnation, he was a middle-aged agent who was hiding the fact he was over 40, the mandatory retirement age for such agents. His main incarnation is as a British U.N.C.L.E. agent in his early 30s.In the first version, Slate had been a mentor to Napoleon Solo (The Moonglow Affair on The Man From U.N.C.L.E.). In his primary version, he works mostly with April Dancer, the first woman enforcement agent for U.N.C.L.E. Mark has a big brother type relationship with April and there is no sign they have ever been romantically linked. Slate also works with other agents, including one assignment with Illya Kuryakin (The Galtea Affair on The Man From U.N.C.L.E.).Mark is a mostly competent operative. He has made the occasional mistake. On one assignment (The Danish Blue Affair), three fishermen -- totally untrained and hardly expected to fight like an agent -- sneak up and overpower Slate.The agent also seems to suffer from bad luck compared to some of his fellow agents. Slate, for whatever reason, is prone to getting beaten up by groups of Thrush agents (The Dog-Gone Affair).