From the synopsis of Balance of Terror at startrek.com:
The U.S.S. Enterprise pursues the warbird, hampered by the fact that the Romulans have constructed an invisibility screen which shields them from view. While the screen protects the Romulans from detection visually, it also prevents them from using their weapons or visual aids. The Romulan Commander, therefore, isn’t sure whether his radar is detecting a Federation ship in pursuit or a harmless space echo.
These are the difficult choices a starship captain must make. Between use of the cloaking device… and powerpoint.