Part 1 (Keith)

After a big fight with his team, Lance leaves Team Voltron to go back to Earth, where he thinks he can make a bigger difference.
Immediately after the events in season 2.
The title is based off an episode title of Avatar: Last Airbender, "Zuko Alone".
(Season 2 spoilers, of course)
(Why am I wasting my life writing fanfiction)