Star Trek: Assignment: Eternity

Captain Kirk first encountered Gary Seven on twentieth-century Earth. Now
Seven, a time-traveling operative for unknown alien forces, makes a
surprise visit to the U.S.S. Enterprise. Kirk is on an urgent
mission to bering relief to a disaster ravaged planet, but Seven has an
agenda of his own -- and he's not above hijacking the Starship Enterprise
and sending it on a perilous journey deep into the heart of the Romulan
Empire.

Kirk must trust Gary Seven once again, as he confronts the
possibility that the enigmatic stranger may bring death and destruction to
Kirk's own era.