Gilbert is a middle aged novelist who has developed a taste for chloral hydrate and bromide washed down with creme de menthe to ease his rheumatism and insomnia. This alongside his regular intake of brandy, gin and wine get him through his days and nights and uses the mixture to make Christmas spent with the children more endurable.  But feeling at odds with life he decides he needs a cruise “anywhere hot, luxury preferred” to restore his “bad nerves”. He is probably clinically depressed at this stage. Various events happen on board to disturb his peace of mind yet which non of the other passengers can perceive. He suffers auditory hallucinations and weaves these into an intricate delusional belief system whereby other passengers (unseen) are trying to harm and ridicule him via a russian thought machine which uses low frequency radio waves through the ships wireless system which he can pick up from various walls in his cabin and the ship. These voices are brought about by the authors liberal use of chloral hydrate, brandy, creme de menthe and other substances taken ostensibly as medication. Thus the drug induced paranoid auditory hallucinations and delusional system come to haunt him as he engages with them in a battle for supremacy as they p[lot against him, accusing him of  what to Waugh were such unspeakable things as being a coward.

In reality it transpires that this was a near lifelike account of Waugh's own experiences on a cruise as his alcoholism and use of phenobarbitones conspired to give him auditory hallucinations. Ultimately for Gilbert upon returning home to his and tolerant and long suffering wife, his doctor merely tells him it was all a case of chloral hydrate poisoning.  The difficulty the reader has is in distinguishing between alcoholic hallucinosis and psychotic illness, both of which this could be.

See http://www.guardian.co.uk/books/1999/mar/20/classics.evelynwaugh and

http://www.bmj.com/content/337/bmj.a2791.full for other useful summaries.