Berthella plumula is an oval-shaped sea slug with an internal shell, which can be up to long. The body is up to and has a cream to orange colour and often displays reticulate markings. The head is flat and a large oral veil lies between the propodium and the mantle. The rhinophores are protruding and enrolled. The species has acid glands in the skin which secrete sulphuric acid for protection in case of danger.
The species is hermaphrodite and the two individuals reciprocally fecundate each other by exchanging their sperm. The reproductive period occurs in spring. The laying is tube-shaped and the eggs are displayed in spiral.