A Romulan warship tests the Neutral Zone in a tense submarine-style duel with the Enterprise.
A Romulan ship crosses the Neutral Zone and destroys several outposts. The Enterprise gives chase in a tense duel of silence and strategy, much like a submarine battle, with both commanders waiting for the other to make a mistake. The crew also faces prejudice when it is learned that Romulans resemble Vulcans.
This episode introduced the Romulans and their cloaking technology, and it features Mark Lenard as the enemy commander, who later played Spock's father Sarek. Many fans rank it among the finest of the series for its respect between opponents.