On the night of September 19-20, 1961, Portsmouth, NH couple Betty and Barney Hill experienced a close encounter with an unidentified flying object and two hours of “lost” time while driving south on Rte 3 near Lincoln. They filed an official Air Force Project Blue Book report of a brightly-lit cigar-shaped craft the next day, but were not public with their story until it was leaked in the Boston Traveler in 1965. This was the first widely-reported UFO abduction report in the United States.
First, stating definitively that the object was a UFO is wrong. There is no evidence that the object every existed, let alone flew. The plaque also leaves out essential details: The Hills claimed to have seen a UFO in their September 20, 1961 report to Project Blue Book but did not claim alien abduction until Betty Hill began having nightmares weeks later (not the next day, as the plaque implies). The Hills' recalled the event under a type of "recovered memory" hypnosis now recognized as almost entirely unreliable, and many of the details are identical to details in movies and TV shows the Hills had watched only days before their hypnosis session, including an episode of The Outer Limits ("The Bellaro Shield"). A fuller discussion is available on the Skeptoid web page for the Hill abduction.
The bigger issue at stake here is the fact that New Hampshire is giving the imprimatur of of the state to an issue that is demonstrably false. The wording of the plaque, though legalistically careful, leaves the undeniable impression that New Hampshire is affirmatively stating that UFOs exist. This is the same state that (in private hands, not through official channels) made a group of colonial root cellars and some boulders into America's Stonehenge and promoted the myth of Irish Celts visiting ancient America.
Acts like a plaque commemorating an alien abduction twist history, enshrine falsehoods as facts, and trivialize actual truth at the expense of self-promotion. What will the historian or the teacher say to the student who asks how UFOs can be fake if the government has endorsed them as real? Worse, how can we judge what is true when those entrusted with curating truth willfully surrender reality to those who would re-create reality to match their fantasies?