Book 4 in which Millie has fun in a sea of electrons is an introduction to electricity. It starts with Millie discovering that her little friends are called electrons because they carry an electric charge. At first she is scared of anything electric, but she ends up having an exciting adventure. She follows the electrons as they move around a circuit in an electric current.

In this adventure Millie learns about electric charges, electric fields, electric conduction, and phonons.