The alien is not hostile nor will ever be. It asks you to call him by the name of BOB. It says it has found a "suitable" partner for you that matches your personality and preference of physical attractiveness. The girl is waiting in the spaceship already. If you say no to BOB, your memory will be erased and you will both return to your normal lives. But if you accept, you leave immediately and will never return to Earth. BOB says two candidates are only given this chance every 50 years in Earth years and only according to age, health and acceptance and open mindedness. The point in this is Mystery and the real question is would you be willing to leave behind your life on Earth to see the Universe?
NOTE: There will NEVER be any ANAL PROBES or anything related to this. If you already have a girlfriend or wife then just pretend she is on the ship waiting.