The Loobenfeld Decay

Summary
Leonard hears Penny singing and it is not good. When she asks him to come to a showcase he lies and says that he and Sheldon have plans. Sheldon is uncomfortable with the lie which Leonard gives and so comes up with a new one. Each time Penny believes him he finds new holes in his own lie and new ways to patch it up.

Critics
"The focus on one story gives the episode a clear structure and a memorable identity. It is both funny and plausible but is not a classic episode of comedy. The humour just isn’t strong enough to match the clever story." - The TV Critic's Review