A metal capsule only half a meter long with an electronic locking mechanism on it is discovered in a smoking crater. Radiation in the area is high, but the crater is far too small to be a nuclear detonation. The Foundation takes the capsule and runs some initial tests, during which the capsule is opened. A brilliant blue light emits from inside as several objects, each too large to fit in the capsule, are expelled from within. The capsule then snaps shut. The items are a collection of strange electronic devices, an engraved metal tablet, and a strange animal preserved in a transparent block of something like acrylic. The metal tablet is discovered to be a key to deciphering an alien language. The translation of this tablet reveals the function of many of the devices, one of which turns out to have a recorded message on it. The message consists of simple text explaining that the capsule comes from a mysterious race from another universe entirely. Many such capsules have been sent to various universes in hopes of discovering another intelligent race, as the galaxy in aliens inhabit appears to be devoid of intelligent life that did not go extinct eons ago. They send out greetings and an invitation to enter a dialogue and exchange of technology.
We eventually discover that this race is not from another universe at all. They are from millions of years into our future. They are our descendants. We killed off all other life in the universe, and then nearly destroyed ourselves. Then a new race arose from the ashes of humanity's funeral pyre, only to contact their forefathers through accidental time travel.