One unfortunate day in a parallel universe at a lab, an accident causes a massive explosion that blows a janitor (working there part time while going to college) out of his world. His anchor to the space-time of his universe is (painfully) severed, and he lands somewhere in USA.
His arrival is also accompanied by an explosion, though unlike at his departure it is merely big enough to be easily noticed. Nobody else is injured by that explosion, but he is found, severely wounded and in a coma, and taken to a hospital.
Even though the wounds should have been easily fatal, he survives with little assistance. After a month, he finally wakes up. While he was unconscious, the government realized that nobody knew who he was—he didn't match the description of anyone missing, he didn't match with any known records of people, and even other governments failed to determine who he is. When he wakes up, the wild theories of him coming from another world gain credibility when they find he speaks a completely unknown language.
It doesn't take him too long to learn enough English to communicate—he is quite intelligent and his memory was significantly improved by the accident. He is also able to figure out that he's no longer on the same planet by seeing a globe and noticing that the continents are not what he is used to. Unfortunately, being a janitor, he has no clue how the explosion transferred him to another world.
By this point he's become somewhat of a celebrity. He is proof that parallel universes exist and that travel between is possible. Everyone knows who he is. With no way home, he tries to adjust to life here. He is given a scholarship to go to college to study physics, where he hopes that he might be able to figure out how to get home. After graduating, he works in a government lab and continues working on ways to go to, or even just contact, parallel worlds. After a decade or so (and no luck getting home), he realizes the final result of what the accident did to him—he's not getting older.
So now the government has access to someone who recently became immortal. They have no means of replicating the process by which he became immortal, so he's the only one they'll ever have. He has an excellent memory that hasn't been (and won't be) diminished by time, and he is very hardy. What would be the best way (for the near future) for the government to utilize someone who is immortal?
For example, if the Earth was going to be destroyed and a generation ship was sent out as humanity's last chance, he would be great to have along because he would not forget the purpose of the generation ship.