Geordi La Forge sets out for a vacation on Risa, but his shuttle is apprehended by Romulans who hope to use him in a plot to drive a wedge between the Federation and its Klingon allies. La Forge returns to the Enterprise with no memory of the encounter- nor of the fact that he's just been exposed to days of mind control techniques that have turned him into the perfect assassin.
This episode was inspired by an episode of "Voyage to the Bottom of the Sea", which involved one of the series' lead characters being kidnapped and brainwashed by enemy agents who intend to use him to subvert a strategic mission and assassinate his commander.
Chief Engineer Geordi La Forge of the Federation starship Enterprise, en route to the planet Risa for shore leave in a shuttlecraft, is captured by Romulans. While an impostor that looks like La Forge is sent to Risa, the Romulans tap into La Forge's visual cortex via his visor sensors, giving them a limited form of mind control over the human. After several days, La Forge's mind is wiped of his capture but given memories of going to Risa, and put back aboard his shuttle, to return to the Enterprise. He arrives as the Enterprise are working with the Klingon Ambassador Kell to deal with rebels attacking the outlying Krios colony. The Governor of the colony, Vagh, asserts that the rebellion is being aided by the Federation, thus requiring the Federation's presence to resolve.
At the colony, Vagh shows Captain Picard and Kell several Federation weapons and medical supplies taken from the rebels. Picard orders his crew to investigate. Commander Data finds that strange E-band radiation is nearby but cannot detect the source. He and La Forge also discover the apparent Federation weapons were replicated using Romulan technology and powered by Romulan energy cells. Later, under the direction of his controllers, La Forge unknowingly transports a case of Federation weapons from the Enterprise to the rebel base, and then subsequently erases the logs. Vagh, monitoring the transport, immediately accuses the Enterprise of its deception. Data and La Forge review the transport logs but find no evidence for the transport even though the weapons originated from the Enterprise, and realize that only they themselves and two other crewmembers could have falsified the logs in that manner. Again, outside of his control, La Forge enters Kell's quarters on the Enterprise, where it is revealed that Kell was the person sending the E-band transmissions used to control La Forge. Kell orders La Forge to assassinate Vagh in a public setting before witnesses in such a manner as to utterly convince Vagh's people of Federation involvement.
At Kell's suggestion, Picard invites Vagh to come aboard the Enterprise as to witness the transport logs first hand. As Picard takes Vagh around the ship, Data comes to learn that the E-band radiation is coming from aboard the Enterprise and that La Forge never made it to Risa. He orders Security Chief Worf to immediately detain La Forge. The assassination attempt is blocked, and Data arrives to explain the situation, saying that the limited transmission range means the device controlling La Forge must either be in Picard's or Kell's possession. Kell refuses to undergo a search, but Vagh offers to take him to the colony to do so there. Fearing the consequences of being investigated by his own people, Kell quickly requests asylum aboard the Enterprise, which Picard says they will certainly consider after his name is cleared of any wrongdoing by the Klingons. Kell is taken away by Vagh's guards. La Forge is cleared but struggles to understand what happened to him.
This is the first episode to feature former castmember Denise Crosby in a role other than that of Lt. Tasha Yar, this time portraying the Romulan Commander Sela. In this appearance she is heard but not explicitly seen, foreshadowing the character's appearance in the forthcoming "Redemption" episodes. She previously appeared as Tasha Yar in an alternate reality in the Season 3 episode "Yesterday's Enterprise".