Humankind was definitely unprepared when the aliens came to the Solar system... Three of their strange ships made of emerald crystal appeared on the lunar orbit within a strange energy outburst. The Earth was frozen with surprise and the aliens stayed silent. For a time. After a few hours they started sending signals in all the languages of the Earth, asking for volunteers to be send to them, to be tested and selected for a travel to the stars so they could learn. Before the wary leaders could make their minds and answer the request, the aliens teleported an asteroid to a moderately stable orbit around the moon and shaped it into a space station. That display of technological superiority and sheer power available to the aliens pushed the decision-makers to ignore doubters and scaremongers and to organize expedition to the moon. They had little choice really, for the private entrepreneurs were already preparing to send their volunteers for the meeting with the aliens, with or without governmental permission.
After a few very frantic months of preparations, the rocket carrying the first volunteers was launched. The world waited in the anticipation as they reached the lunar orbit and docked at the alien base only to discover that the station was clearly shaped with humans in mind. There was nothing revealing the appearance or nature of the aliens, nor was there sign of truly alien technology. Everything was build using technologies already known to humans, albeit with blatant disregard for costs—using the most cutting edge materials and the most rare minerals as needed to achieve the best possible results. The aliens were nowhere to be seen either, communicating with the astronauts through the computers of the station. The hosts thanked the volunteers for coming and selected a few of them to follow the light signals until they reached inner dock where they found what was clearly some sort of starship. The rest of the volunteers learned that they were found physically and mentally unfit for the incoming journey; the aliens offered them a ride home using shuttles that were built along with the station or a place to live on the station. In mean time, those who were found fit boarded the ship and launched. The world and the remaining astronauts then viewed as the alien ships surrounded the vessel and disappeared in the burst of energy.
The vessel, like the station appeared to be constructed using cutting edge Earth technology, using prohibitive amounts of exotic materials and prototype technologies. Its computer database is filled with all the technological knowledge of the humankind. And yet, the available plans show a blank spot inside the hull, inaccessible part connected to the rest of the ship through fiber-optic lines. Is that the FTL Drive? The heart of the very helpful, childish, and unable to provide actual answers artificial intelligence that directs the ship?
The aliens inform the travelers that their vessel is currently many parsecs from the Earth and that they have resources and means to explore the galaxy, learn new things, and develop their ship and themselves further beyond the limitations and restrictions of the planet-bound humans. The ship has power source, computing power, and drive they could not dream of, and everything else is up to them...
And then they vanish again.