Ponytailed and potbellied, Renatus is a teacher of mathematics and physics who still lives with his elderly mother; he also has a not–so–secret obsession. For Renatus, co–sinus curves and radius vectors revolve back to one thing: female bosoms, especially the "perfect" bosom of famous German actress Senta Berger. Seidl steadfastly follows Renatus on his daily routines: tracing bosoms on his classroom chalkboard, delivering Berger bosom slideshows in the dark, standing around his cramped apartment half nude, and continually battling a mother seemingly poised between the couch and the grave. Meanwhile, shoved somewhere between the toilet and the id, Seidl's camera whirls away, exposing every uncomfortable fight, every obsessive rant. Illicit and truly disturbing, like peeking into your oddest neighbor's door or being caught inside someone else's diary, The Bosom Friend reveals fantasy and reality as an obsessive female ideal and a possessive mother, the void between them lonely and loveless.