The lesson plan teaches the difference between proper and common nouns. It begins with a review of nouns and a dialogue to motivate students. Students then underline nouns in the dialogue and discuss the differences between proper and common nouns. The teacher explains that proper nouns begin with capital letters and name specific people and places, while common nouns begin with lowercase letters and name general people and places. Students then practice identifying proper and common nouns. The lesson emphasizes the importance of using capitalization correctly with nouns.