When the boys go to eat at Penny's restaurant, she learns Leonard has never had a birthday before. She decides to throw him a surprise party and blackmails Leonard into helping.
Sheldon initially refuses to buy Leonard a present until Wolowitz convinces him and Penny drives him to the computer store where he just ends up being distracted and helping customers with their tech support problems.
Meanwhile, Wolowitz is in charge of keeping Leonard in distracted until the party. Wolowitz pretends to eat a peanut bar (to which he is allergic) and makes Leonard drive him to the emergency room. However, due to Sheldon, Wolowitz needs to keep Leonard even more distracted. After a promise from Penny to show Wolowitz which of her friends are easy, Wolowitz eats an actual peanut bar and and has a real allergic reaction.
After spending several hours at the hospital, Leonard and Wolowitz return to the apartment to find Penny and Sheldon sleeping and Koothrappali singing Karaoke. Apparently everyone already left over an hour ago. Penny shows Leonard the party on a cell phone video and gives him a kiss.