Chibi-Usa tricks Ryoga into giving a time key to Ranma in order to get Ranma to meet Pluto at the gates of time and take her position as Sailor Charon and guard the gates as well. This is to save Ranma from death at the hands of his fiances in the future. Ranma was shown this future and he has to act the same in order to motivate Chibi-Usa to travel back in time to try and save him. (Sailor Moon Crossover.)