Penny, with the help of the rest of the gang, are throwing Leonard his first birthday party ever. Howard needs to keep Leonard away from his appartment, Raj needs to "entertain" the guests when they arrive, and Penny needs to help Sheldon buy a gift after he finally accepts the process of gift giving.
Complications arise however when Sheldon gets a little too into the products he's sampling,
Great character development happening here for Leonard. We learn about his childhood and sympathesize with him, and we get our first Penny/Leonard kiss!