A high endurance mobile unmanned aerial vehicle system includes a base station and an unmanned aerial vehicle interconnected by a tether which is releasable from the unmanned vehicle. The unmanned vehicle receives electrical power from the base station to operate the vehicle while connected. The unmanned vehicle further includes a hybrid drive system operable to produce electrical power from a fuel carried by the unmanned vehicle such that the unmanned vehicle can start the hybrid drive system, release the tether and fly in a fully mobile mode, away from the base station and tether when desired.