Dick gets the sack and Duncan resigns in protest. With Collier gone and the Binghams bringing up triplets they've nothing to stay around for and become ship's doctors on passenger liner the Begonia, only to find the captain is Sir Geoffrey's twin brother Norman. Next day they are shipped aboard by a crane to a very dirty ship whose last doctor died of suspected parrot fever and where they are likely to be pursued by crewman Mad Jake. Fortunately they find out in time they're on the wrong ship, it's not the Begonia after all.
Having put to sea, Duncan discovers that he is prone to sea-sickness so Dick gives him a healthy dose of remedial medicine. Unfortunately he overdoes the dosage and Duncan falls into the ship's swimming pool, arriving soaking wet at the captain's cocktail party. The captain sacks him and in revenge Duncan tells American passenger Mrs. Tranmere that Loftus loves to sing, so she asks him. Ultimately Loftus reinstates Duncan because the party was a huge success, due to his singing.
There is a loss of manpower on the Begonia because Dick has been doling out sick notes or chitties to the crew. Captain Loftus discovers this and threatens to sack him. Industrial action is called. However, when the ship arrives in Madeira nobody wants a chittie, as it would mean missing shore leave. Dick has been fooled by the crew. He has, however, made money by selling the chitties at two pounds a time.
Dick and Duncan are both smitten by the lovely Anna-Maria, a glamorous Austrian passenger. Dick gets to bandage her finger but Duncan,as the senior officer, takes over and keeps Dick away from her by forcing him to make animal noises in a talent contest. Anna-Maria is persuaded to enter a beauty pageant which she wins. However she has also attracted the attentions of the captain and when Duncan goes to her cabin, finds Loftus there in a compromising position.
The purser finds a pair of trousers with the name tag 'Manuel Sanchez' on and knows there is a stowaway on board. Dick finds him hiding in his wardrobe. He tells him he came to England for work but missed his girl-friend in Tenerife. Then Duncan finds him and gets a different story. The two doctors finally work together and, at Tenerife, dress Manuel as a souvenir vendor, knowing the purser will throw him off the ship as he wants.
Dick is running a late night poker school and a money exchange for passengers but collapses from the stress of it all and needs bed rest. He gets Duncan to take over but Duncan fumbles the exchange, giving the passengers overly generous rates and is hopeless at poker, losing to Captain Loftus, who finds out about it and wants to be included. Finally Duncan collapses from exhaustion - as does the purser.
Dick and Duncan have already been carpeted by Captain Loftus for being late back on ship after shore leave but now find themselves stranded on Madeira as the Begonia has sailed without them. They get hold of a small motor boat to chase after their ship and eventually wind up back on board but only after Dick has knocked Duncan overboard.
Duncan is dismissive of Mr. McArthur in front of fellow passenger Polly Roberts, an undercover journalist writing an article on the cruise. She complains about him to the captain, who tells him a Very Special Passenger has found fault with him. Duncan assumes this is McArthur and keeps out of his way. When Polly gets food poisoning, thanks to dodgy squid Dick bought for the captain's table, she is charmed by Duncan's concern and they bury the hatchet. Knowing she is the undercover journalist he is again rude to Mr. McArthur - who turns out to own the ship.
The two doctors are relaxing on a Spanish beach when Duncan sees a local girl in difficulties in the water and rushes to rescue her, applying mouth to mouth resuscitation. The locals assume he is kissing her and Duncan finds he is expected to marry her. When he refuses he is accused of rape and jailed, requiring Captain Loftus to bribe the police chief in order to release him.
Whilst Dick beats the captain at cards, Duncan has a day on the beach with the charming Rosita, only to find himself in extreme agony, clutching his side. He is sure he has appendicitis but, now the Begonia has sailed, there is only one other person on board - discounting the vet - who can operate - Dick Stuart-Clark. Duncan is most reluctant to let his friend loose on him though fortunately it transpires that his affliction is food poisoning from the ice cream he ate on the beach.
Mrs. Tranmere, an influential middle-aged American woman, has taken a shine to Duncan and the Captain orders him to dance attendance on her to keep her sweet. Duncan is more interested in her daughter, who has reciprocated the feeling, and he has also arranged to go on a date with her. To add to the complications, Joyce, the ship's nurse, gets drunk and throws herself at the doctor, requiring him to run from one date to the next, finally ending with none.
Duncan is sick of the purser offering medical advice and the purser is anxious to rat on Duncan to the captain for taking bribes from the kitchen. After the couple have publicly come to blows, the purser disappears and Duncan is suspected of killing him. The purser reappears, having got locked in a store room and escaped through the ventilation shaft, and the feud continues.
The Begonia comes home after the cruise and Duncan, having fallen out with Dick, decides he has had enough of being a ship's doctor and returns to St. Swithins. Things have changed but Sir Geoffrey will have him back - and Dick too. Should they accept? Or should they return to the Begonia?