Balance of Terror

While investigating a series of destroyed outposts, the Enterprise discovers a lone Romulan vessel with a cloaking device. The Romulans, having never been seen by humans, are revealed to visually resemble Vulcans, which causes navigator Lt. Stiles to suspect Spock of treason. The two ships become locked in a cat-and-mouse battle through space, leaving the Enterprise seemingly crippled by Romulan weapons.[37]