They must send Jenny and Silverthorn back to their respective times, in order to restore the timeline. Silverthorn, having faked remorse for his crimes, has surreptitiously stolen instructions for building the Time Gate.
In order to revive the timeline, they need to dispatch Jenny and Silverthorn back to their respective epochs. However, Silverthorn, who had feigned regret for his wrongdoings, has covertly obtained blueprints for building the Time Gate.