Penny asks if Leonard is going to come to The Cheesecake Factory for birthday cheesecake, but Leonard tells Penny that he's never properly celebrated his birthday. Upon learning this, Penny hatches a plan to throw Leonard a surprise party.
On the big day, Penny accompanies Sheldon to an electronics store so that he can buy a present for Leonard. Raj hangs back to set up the party, and Howard is entrusted with distracting Leonard until everything is ready. However, Sheldon dawdles at the store and is accosted by a number of customers who want his help; meanwhile, Howard is forced to eat peanuts to get Leonard out of the apartment.
When the ER nurse finds out Howard faked the peanut reaction to get Leonard to leave, she wants them to get out of the hospital, but it is at that time that Howard hears from Penny and knows he needs to keep Leonard away for longer. He then actually eats peanuts to get himself escorted into a doctor's office.
They spend so long at the hospital that by the time Leonard and Howard arrive back at the apartment, the party is over. Only Raj yells, "Surprise" in the middle of an off-key karaoke song.