Leo was at home when he fell asleep. When he woke up, he had become one of the Elders of the Universe's Collector's exhibits. Before he could even figure out what was going on, Deadpool came along and broke him out. To escape, Leo freed all the creatures imprisoned in the museum, then used the chaos to loot the Collector's vault. White Tiger Amulet, Fuxi Sword, Element Gun, Qiankun Ring. What's yours or his? I picked them up, so they're mine. He also gathered a team to help one another get back to their own universes. Little did he know that this would drag him into even more major events.
Leo: "I just wanted to go home. How did I end up as the leader of these superheroes?"