"There are 20 letters in the alphabet, correct?"
"No, it's 26."
"Oh, I forgot, you are a cutie."
"You're missing one more."
"I'll give you the D later."
"....come to my office at 1pm ASAP."
Alphabet Antics
Explanation
Explain Bear
Listen up, buttercup. The joke is that the person first says there are 20 letters in the alphabet, then implies the other person is a "cutie", but that wasn't the missing letter. The missing letter is "D", as in, I'll give you the D later which is slang for intercourse. Get it? You probably don't.