Season 1

The four nerds are on a visit to Cheesecake factory when they become like a little boy celebrating his birthday Because of what Penny said, Leonard says that he also has a birthday soon and how he would like to celebrate this one. To her surprise Leonard confesses that he'd rather not celebrate his birthday, because he has never celebrated it before, and also never had a birthday party. Not even as a little boy. After thus fascinating story Penny decides to throw a surprise party for Leonard. The tasks are quickly distributed. Raj prepares the flat for the celebration and is to keep the guests entertained until the birthday child comes. Howard was given the task to keep Leonard out of the apartment until everything is ready to start. And Penny goes together with Sheldon to buy gifts.

Later in the electronics store Penny persuades Sheldon to throw overboard his attitude towards gifts and buy his best friend a gift. Meanwhile Howard tries to get the birthday boy out from the appartment. Leonard is very busy measuring his HALO skills with a sick boy. Since all Howard's attempts failed, he must resort to a white lie and pretends to have eaten a Granola Bar which contains peanuts. Since Leonard knows about Howard's peanut allergy and is aware of the seriousness of the situation, he drives the doomed-to-die directly to the hospital.

As it gets more difficult for Howard the mimic his symptoms, he calls Penny to ask her how much time they still need. Since, Sheldon has become the seller and is serving a customer, Penny explains to a rather baffled Howard, and that they are still stuck in the store and still need more time. When she learns what position Howard is in, she promises to set her up with one of her girlfriends. Spurred on by this prospect, Howard really eats a piece of the cereal bar which provokes an allergic reaction.

After the doctors have ensured that poor Howard won't suffer further and have supplied him with medicines, the two make their way home. When they are at home the two find that the party is already over. There, Raj has filmed but everything, and Leonard and Penny watch the night's events on video. After the video is over, he receives another 'Happy Birthday' from Penny and the probably best birthday gift that he had: A kiss on the mouth.