The Starship Enterprise is sent to persue a ship that has destroyed Earth Outpost 4; near the Romulan Neutral Zone. It is feared that after a century of non-contact, the Romulans have built up their forces again. Kirk gets there and discovers a lone ship, that has an ability which surprises the entire crew, Mr. Spock included - it has a cloaking device.
The Enterprise finds that this is not perfect and manages to track the ship - it is returning home after what appears to have been a test of strength. The Enterprise taps into the Romulans internal security cameras and makes another astonishing discovery : the Romulans are, in appearance, identical to the Vulcans, a member world of the United Federation of Planets. Mr. Spock denies all knowledge of this.
The episode ends with the Enterprise finally cornering the Romulan Bird of Prey; after a battle, the Enterprise is victorious and offers to take on survivors: but the Romulans self-destruct, rather dying than being taken prisoner.