The primary purpose of this article is to determine the impact of a simulated agent's transparency on human performance and related variables, such as response time, workload, and trust calibration. The agent supports participants as they complete a base defense task by managing a team of heterogeneous unmanned vehicles and serves as a decision aid to the human. Three conditions of transparency are explored. In condition 1, the agent displays only the basic information (map of vehicle location and proposed routes). In condition 2, the agent displays the basic information and an explanation of its reasoning. In condition 3, the agent displays the basic information, reasoning, and uncertainties involved in the plans. Results show that participants exhibit better performance and trust calibration in the high-transparency conditions without perceiving a significant increase in workload. However, response time also increased, likely due to the additional processing time needed for conditions with more information. Overall, our findings indicate that the increased agent transparency can improve human-agent decision making and performance, but with a small cost of the efficiency (timeliness) of task completion.