A Satarran in disguise
The Satarrans are a humanoid civilization from the planet Sothis III. Although they are not as technologically advanced as the Federation, they possess a unique device which allows them to wipe the collective memory of the entire crew of a starship. By 2368, the Satarrans had been at war with the Lysians for decades.
In that year, the Satarrans attempted to use the USS Enterprise-D to launch a devastating attack against the Lysian Central Command. They used their memory wipe weapon on the crew, placed an operative known as Keiran MacDuff in the first officer position, and planted false orders from Starfleet Command which stated that the Enterprise was to destroy the Lysian headquarters. Although the crew was unable to restore their full memories by the time they were to attack the Lysian headquarters, Captain Jean-Luc Picard realized that the Lysians posed no threat to the Galaxy class starship or to the Federation at large, and that the orders must have been falsified. MacDuff's true nature was then revealed, and the Enterprise stood down. (TNG: "Conundrum")
Satarrans allegedly hate mysteries. (TNG: "The Chase")
Background information
The Satarran head and torso were designed and sculpted by Michael Westmore and Ed French, taking four days to complete the sculpt. For its brief appearance, the head and torso were mounted on a stand, shot frame-by-frame, and inserted optically into the final shot. Westmore considered it "one of the most time-consuming and expensive makeups we've ever done [...], and yet it was only seen for a few seconds". (Star Trek: The Next Generation Makeup FX Journal)