It explores the processes of creativity through a portrayal of an ageing novelist, played by John Gielgud, who imagines scenes for his latest novel which draw upon his past and his relationships with members of his family.

On the eve of his 78th birthday, the ailing, alcoholic writer Clive Langham spends a painful and sleepless night mentally composing and recomposing scenes for a novel in which characters based on his own family are shaped by his fantasies and memories, alongside his caustic commentary on their behaviour.

On the following day, Clive welcomes Claude, Sonia and Kevin (in reality an astrophysicist) for an idyllic birthday lunch in the sunlit garden of his country mansion, and their relationships are characterised by mutual affection and good humour, albeit with signs of self-restraint in deference to the occasion.

Resnais held a longstanding ambition to cast John Gielgud in a substantial film role, having seen him performing on stage, and was encouraged to approach him by Dirk Bogarde.

[1] Resnais attached great importance to the interplay of vocal timbres of his principal actors, and he described how he thought of them as a Schubertian quintet: Ellen Burstyn a violin, Dirk Bogarde a piano, David Warner a viola, John Gielgud a cello, and Elaine Stritch a double bass.

"[17] In counterpoint with creativity, the theme of death recurs constantly, not so much as a subject in itself but in Clive's struggle to avoid it: Resnais described the film as telling the story of the old writer's determination not to die, and his continual drinking and imagining are the evidence of his refusal to let go.

[18] As well as the funereal aspects of the decor and the scenes of autopsy, the repeated instances of metamorphosis of a character into a werewolf is linked to the advent of death, with the implication that the process of dying reduces man to animal.

[18] Other motifs which contribute to the mood of morbid anxiety are the military search parties and images of deportation, the helicopter surveillance, the sound of bombs and ambulance sirens, and the demolition of buildings.

[20][21] As several writers about the film have observed, the opening sequence mirrors the beginning of Citizen Kane: the plaque outside the house, the camera closing in on a lighted doorway, the breaking of a glass object, the close-up of the lips of Clive as he curses.

[28] Vincent Canby in The New York Times called it a "disastrously ill-chosen comedy" and "a lot of fuss and fake feathers about nothing"; he found the script pretentious and the structure complicated without being complex.

[30] Pauline Kael wrote a 2000 word review in The New Yorker which found fault with the contradictory structure, the stilted language, the artificiality of the acting, and the glacial directorial style of the film before concluding that all it amounted to was "the pain of a 'clever' English play".

[24] Gilbert Adair in Sight & Sound contrasted David Mercer's excessively literal script in which "nothing is left unstated" with the extent of the personal mythology and fantasy which Resnais was able to introduce into the film; he found the work enriched by its anti-naturalistic devices such as the gaffes in continuity which emerge in Clive's plotting of his novel and the exchange of voices of the characters, as well as by the disjunctive appearances of a clownish footballer in inappropriate scenes; and despite certain reservations he concluded that "the dream cast perform together superbly".

[21] A specific criticism of one aspect of the film appeared in a comment column of the British Medical Journal, where it was argued that the inclusion of scenes of a post-mortem on a corpse (accurate but unsparing) was "undignified and uncivilised and ought to be condemned" because the audience was not prepared for them and they were unnecessary to the plot.

[36] For the critic Jonathan Rosenbaum, "The superb performances and Miklós Rózsa's sumptuous Hollywood-style score give the film's conceit a moving monumentality and depth, and Resnais' insights into the fiction-making process are mesmerising and beautiful.

[31] Others have found different grounds for criticism, arguing that the final 'real' section represents a compromise and a concession to conventional cinema, a denial of all the bold experimentation of the previous four-fifths of the film.

[39] Alain Robbe-Grillet, writer of the screenplay for Resnais's earlier exploration of imagination and recollection, L'Année dernière à Marienbad, was one of those who disapproved of the final section.

[40] An alternative view is that the final lyrical section of the birthday party does not present a definitive picture of the family as they really are, but rather another perspective on them in the puzzle which Clive – both as writer and as father – is trying to solve.