The Valiant - Michael Jan Friedman

The USS Stargazer, a long-range exploration ship commanded by Captain Ruhalter, is assigned to pick up descendents of the survivors of the USS Valiant, a ship that was lost at the far edge of the galaxy in the very first Star Trek Original Series episode. The evacuees claim that a deadly invasion force from outside the galaxy is nearing the United Federation of Planets. When they are ambushed by the invaders, Captain Ruhalter is killed, leaving a young Lieutenant, Jean-Luc Picard, in charge of the ship in the midst of a deadly confrontation.
Picard must swiftly learn to be an effective Captain; while dealing both with a crew that has not yet come to trust his judgement and with hostile aliens more powerful than anything the Federation has encountered before.