Berthella plumula appears transparent and fragile with an oval, convex body, up to 6 cm in length. The shell is up to 3 cm long but internal, covered by the mantle which overlaps the shell edges. The mantle itself is smooth, and pale yellow to orange in colour, often having net-like markings down the middle. The edge of the mantle may actually be raised on the right side where an elongated gill is situated, for respiration. Its head has a noticeably flat oral veil with enrolled rhinophores (sensory processes). A conspicuous gland (the metapodal gland) develops at the posterior end of the body at a pre-sexual stage when the species is approximately 1.75-3 cm in length. When in danger acid glands in the skin secrete sulphuric acid for protection.This is a slow moving opisthobranch and a specialised predator; feeding on compound ascidians. Breeding occurs in spring. Spiralled egg ribbons are attached to rocks, with the larva being planktonic.