You see a scene in the future in front of the Leene Carnival Bell. Robo's girlfriend runs right into him just as Marle runs into Crono at the very beginning of the game. The two then watch from a high mountain.

Conditions:
You can get this ending as soon as you go to 600 AD the second time, but when you finish and try to save the system file, the game will think you are trying to save ending 5 instead of 6. So, wait until you hear that Tata is the Legendary Hero, but before you get the Hero's Badge from him, then defeat Lavos.