Star Trek Department of Temporal Investigations - Shield of the Gods

the long version

The stalwart agents of the Department of Temporal Investigations have tracked down many dangerous artifacts, but now they face a greater, more personal challenge: retrieving a time-travel device stolen from their own vault by a rogue agent of the Aegis, a powerful, secretive group that uses its mastery of time to prevent young civilizations from destroying themselves. Blaming the Aegis itself for a tragedy yet to come, this renegade plans to use the stolen artifact to sabotage its efforts in the past, no matter what the cost to the timeline. Now the DTI's agents must convince the enigmatic Aegis to work alongside them in order to protect history–but they must also wrestle with the potential consequences of their actions, for preserving the past could doom countless lives in the future!