One of the more surprising aspects of the early missions of the NX class Enterprise during the 2150s was the number of hostile alien species which the ship encountered. One of the more mysterious of these incidents came in 2151, during the ship's first year of exploration. Enterprise was deploying Subspace Amplifiers to expand Earth's communications network when an alien vessel approached. The ship refused all attempts at communication, simply observing Enterprise for a short time before departing. Over the next few days the vessel returned several times, but on subsequent visits it scanned Enterprise and then launched a series of unprovoked attacks on the ship.
The alien vessel was fitted with a defensive energy screen which Enterprise's torpedoes had little impact upon. The initial haste to get Enterprise out of dock in time to return the Klingon courier Klaang to his home world had meant that there was no time to install the ship's most powerful weapons - three five hundred Gigajoule phase cannon. The engineering and weapon crews managed to install the single cannon which the ship carried in storage, and to construct and install a second cannon. This remarkable feat, which would be expected to take up to a week in dock, was completed in 48 hours whilst the ship was still underway and engaged in periodic combat.
On one of the attacks the alien craft dispatched a shuttlecraft which landed a two person boarding party aboard Enterprise in what seemed to be some form of reconnaissance mission. The aliens were Humanoid, but of a species completely unknown to the crew. They were equipped with personal force fields which proved invulnerable to the fire of the phase pistols Enterprise carried, and after conducting scans of some crewmen and planting a surveillance device the aliens withdrew.
Once Enterprise had its phase cannon installed, they were tested against a small mountain on an uninhabited moon. The effects were unexpected; an overload caused the cannon to fire with ten times the expected yield, completely destroying the mountain and inflicting significant damage on the ship. Some modification allowed the cannon to perform more normally, but when used against the alien craft they proved as ineffective as the torpedoes had.
The chief engineering officer formulated a method of protecting the ship from the overload damage experienced earlier, and Captain Archer ordered that the conditions of the first firing be deliberately re-created. The overload allowed the phase cannon to puncture the shields of the enemy ship, inflicting considerable damage on it. A volley of torpedoes convinced the aliens to withdraw. 
There has been no further encounter with this species since 2151, leading most to speculate that they were only passing through this region of space rather than being permanent inhabitants.