This is a fairly lengthy and not brief story but it covers the whole sequence of events.
Crazy Joe DaVola was someone that Jerry and George met at NBC studios when they were there for their discussions about their "show about nothing" Pilot. Joe also knows Kramer and at NBC Jerry spills the beans about a party that Kramer was having of which Kramer intentionally did not invite DaVola because "he is a nut". DaVola becomes incensed and is after Kramer. When they meet DaVola kicks Kramer in the head and the only reason that Kramer is not seriously injured was because he was wearing the motorcycle helmet that he got from Newman in exchange for the radar detector that was defective.
Joe DaVola is unstable and requires medication or else he acts like a madman. DaVola has run out of his medication and can't get anymore because his psychiatrist is in Europe. The psychiatrist has started up an affair with Elaine who he met in Europe but his concern over how DaVola will react without his medication disrupts their relationship. When they return, Elaine tries to break up her relationship with the psychiatrist by saying that she has started a new relationship with Kramer. At the same time, Kramer tells Jerry that DaVola is after him next and this causes Jerry to live in fear. One time, Jerry thinks he sees DaVola outside of Monks and he and George try to persuade a policeman who is eating there to escort them out.
When Kramer calls the psychiatrist to tell him about his relationship with Elaine, the psychiatrist talks Kramer into coming to his office for further discussion. Elaine goes with Kramer there and when Kramer is talking with the psychiatrist, Elaine waits outside and meets DaVola. They hit it off and a relationship begins. Elaine invites Crazy Joe to the opera Pagliacchi that she is going to with Jerry, George and Kramer.
DaVola leaves a message on Jerry's answering machine saying that he has put a kibosh on Jerry. Elaine shows up at DaVola's apartment unexpectedly. She sees several photographs of her that DaVola has taken with a zoom lens camera without her knowing. She realizes that he is mad and he surprises her and refers to her as Neda a female character from Pagliacchi. She escapes by spraying him with cherry Binaca.
Since Elaine does not want to go to the opera with Joe anymore, outside the opera house in a nearby alley, Kramer scalps the ticket that was to be used by Joe by selling it to a man dressed up like a clown that turns out to be DaVola. While waiting on line to attend the opera Jerry and Elaine both realize that the madmen that they are each dealing with is the same person. When they see the clown, they both smell the cherry Binaca scent and flee the scene. While seated waiting for the opera to start, they ask Kramer who he sold the extra ticket to and he answers a man dressed in a clown suit. Elaine and Jerry both panic and that episode ends without explaining who that situation was resolved.
Later that year, back on medication, Joe DaVola is occupied cleaning windshields and he wishes Jerry good luck on the Pilot while cleaning his windshield. At the taping of the Pilot, Joe is in the audience and jumps onto the stage while uttering the same Latin phrase that John Wilkes Booth said when he asassinated President Lincoln. I don't recall him appearing in the series after that.