Mix cake as directed on box and pour mixture into a 13x9 inch pan. Place spoonfuls of lemon pie filling in blobs over batter.
Bake as directed on the cake box. Let cool completely. Cake batter covers blobs, so there will be pockets of lemony filling inside. No need to frost.
If you want to frost it, you can use 1 can ready to spread lemon flavored frosting with 1 container of Cool Whip. Mix frosting and Cool Whip together and spread on cake. This does need to be refrigerated. Chocolate is also good.