Adapted from Ken Kesey’s novel, the film centres on Randle McMurphy (Nicholson), a convict who simulates mental illness in the hope that a transfer to psychiatric hospital might ensure his early release. But he hasn’t bargained for the rigid regimen of Nurse Ratched (Louise Fletcher, also superb), who dislikes his disruptive — though he’d say liberating — effect on the ward.

Inspired casting (Danny DeVito, Brad Dourif and Christopher Lloyd are among the patients) and Forman’s naturalistic direction lend authenticity to the proceedings, so that the film succeeds both as anti-authoritarian parable and as an affecting reminder of the psychiatric practices of the past.