A party is being prepared in a converted warehouse somewhere in London's East End, hosted by wealthy young jazz fan Rodney Hamilton to mark the anniversary party for the self-styled first couple of the London jazz scene, Aurelius Rex and Delia Lane. Rex's manager Cass arrives and starts planning the opening number with Rodney, though the latter is more interested in hearing about Delia, who has somewhat faded from view since her marriage.

The drummer Johnny Cousin arrives with his prominently self-labelled drums and his somewhat retiring wife Emily. While the other guests (who include Charles Mingus, Johnny Dankworth and Dave Brubeck) arrive, he rings agent Lou Berger to discuss his plan to form a band with himself on drums and Delia on vocals. Berger says that he's only interested if Delia is involved. He returns to the party just in time to strike up the drums as Rex and Delia enter, and once the music is in full swing he asks her to join him - and is politely but firmly brushed off, on the grounds that it would end her marriage.

Cousin approaches Rodney to discuss "our deal", which involves Rodney putting up £25,000 to let the band get off the ground. Like Berger, Rodney is only interested if Delia is involved. Cousin starts to sow the seeds of a rumour that Delia is having an affair with Cass, but is interrupted by Emily. While a visibly shaken Rodney returns to the party, Cousin pumps her for information about Rex and Delia, whom he then observes intently while the band plays.

Rex discusses his marriage with Delia, saying that he dreams about her leaving him and that as a result he never wants to lose sight of her. As Rex gets up to play with the band, Cousin quietly pockets a gold cigarette-case that Delia had left on the table. He then goes to talk to Cass, offering him a drugged cigarette, and spreads rumours about Berger.

While a despondent Cass is consoled by his girlfriend Benny and then by Delia, Cousin talks to Rex, spreading rumours about Delia's mysterious trips to Cass's room for "rehearsals". Rex asks him to stop this particular line of conversation, but is clearly troubled. Cass has a violent confrontation with the newly-arrived Berger before storming out. While Cousin defends Cass's reputation to the rest of the party, Rex follows Cass out and fires him, though it's clear Rex is doing this reluctantly.

While Benny is consoling Cass in one of the private rooms, Cousin surreptitiously switches on a tape recorder, taping the rest of the conversation and Cass's subsequent one with Delia during which she says "I love you, and so does Rex". Meanwhile, Rex is talking to Benny about her relationship with Cass, and is worried to hear that it could be better.

While Delia is singing 'All Night Long' at Rod's request, Cousin edits the tape, removing the line "and so does Rex" amongst other alterations. He then casually alludes to the existence of the tape - recorded accidentally, of course - during a post-song conversation with Rex, but Rex refuses to listen to it. Cousin also claims that Delia gave the gold cigarette case to Cass.

Rex quizzes Emily about how often Cass "rehearsed" with Delia and asks Rod if he knows any gossip. Before he can express his obvious frustration at Rod's denials, Delia comes up to him and expresses her concern about his behaviour. Rex tests her by asking for a cigarette, but Delia has mislaid her cigarette case. Infuriated, Rex goes to Cousin and demands to hear the tape, which now includes such amended lines as "I love you - you leave Rex to me: I'll fix it for you".

An enraged Rex rushes upstairs and tries to strangle Delia, pushing Cass over the balcony when he comes to her assistance. In the ensuing uproar, all of Cousin's schemes are revealed. Rex wants revenge but Delia reins him in. Bridges are built with Cass and Rex and Delia go off together. As the other guests leave, Cousin drums to himself in the corner, wrapped up in his own little world.