The Romulan Stratagem

Summary
From the back cover: On a mission to an unexplored planet near the Klingon/Romulan border, the U.S.S. Enterprise travels to the planet Eloh to negotiate Federation memebership. But upon arrival, Captain Jean-Luc Picard and his diplomatic team find themselves face to face with Commander Sela, an old Romulan enemy -- who's there to convince the Elohsians to join the Romulan Empire.

When a series of fatal indidents casts suspicion on both delegations, Data must form an uneasy alliance with Sela to prove the innocence of the Starship Enterprise crew -- or lose a strategic stronghold to the Federation's greatest enemy.