Like that quote from the US Office, "I love inside jokes. I hope to be part of one some day."
July 2020, an acting troupe learns theaters won't be open post-lockdown, so the play they were rehearsing (Guermantes, after Marcel Proust) will never be staged. Guermantes, the film, is how the group of actors respond to this news by continuing the rehearsals anyway, camping out in the theater, and doing a slew of other things that involve being naked.
The film is much like the works of Proust: over-long, long winded and ultimately pointless. What is no doubt a fun video photo album for the actors is little more than a pile of obscure anecdotes for us, the uninitiated and uninvited audience. A curious waste of time that is a must for theater kids only!