Brock Lesnar left WWE after his disaster of a match against WWE Hall of Famer Bill Goldberg at WrestleMania 20. The fans knew that both wrestlers were leaving after the match, and made it clear that they didn't care about either of them one bit.
After Brock's departure from WWE, he tried his hand at NFL but it didn't go anywhere. Soon, he signed a contract with NJPW, where he wrestled the likes of Shinsuke Nakamura and Akebono.
In Lesnar's autobiography, "Death Clutch", he revealed an incredibly riveting story from the time he signed with NJPW. A long-standing tradition in NJPW was for newcomers to get slapped by Antonio Inoki, who was a living legend in Japan, and had founded New Japan Pro Wrestling in 1972.
Back in his prime, Inoki was one of the most popular wrestlers in Japan, and gained international fame following his fight against Muhammad Ali in 1976, that ended in a draw after 15 rounds. A badass in real life, Inoki entered politics while still being an active wrestler in 1989. He went on to successfully negotiate the release of Japanese hostages in a talk with Saddam Hussein, before the outbreak of the Gulf War.
According to Lesnar, he let the officials know that he wasn't going to be a part of the initiation. He clearly told them that no one was going to slap him.
Another game I was ready for was the Inoki Slap. I guess part of the initiation over in Japan is that all the newcomers end up getting slapped by Antonio Inoki, who is a living legend over there. I let them know where I stood on that one right away. No one was going to slap me. Not Inoki. Not anyone. They got the point.
Inoki was inducted into the WWE Hall of Fame in 2010. Brock Lesnar is currently working in WWE and holds the Money In The Bank contract.