© ITV/Mark Bourdillon

Bullard asks Barnaby to investigate the disappearance of Lucy Oliver, the daughter of some friends. Lucy was a member of the Oblong Foundation, a new age cult based at Malham Hall. Jones returns from leave, with a beard, and Barnaby seizes the opportunity to send him into the cult, undercover, as new recruit Cosmo Jones.


In return for the favour, Barnaby and Sarah ask Bullard to help them secure a place for their dog Sykes with Bullard's sister Millie. She's a dog minder but is very particular.


Ruth Lambert inherited Malham Hall when her parents Jeremy and Carolyn died in a gas explosion aboard their boat. At the local pub run by Paddy and Claire Powell, Barnaby hears various conspiracy theories about how the Lamberts met their death.


At the cult, led by the charismatic Dominic, Jones is befriended by Blaze, who seems equally ill at ease. Jones fears his cover will be blown when he takes Ruth to the village to pick up a letter, and he's even more worried when Ruth seems much taken with him.


Freddie, spokeswoman for the Foundation, is away and Blaze persuades Jones to go snooping in her bedroom under the guise of visiting the 'love room', but they get locked in and have to call on Barnaby for help.


The Lamberts' lawyer Susan Crane reveals her clients withdrew thousands of pounds from their account, supposedly for gambling. But she also gave Jeremy Lambert advice about the use of a promissory note or legal IOU. Meanwhile, Barnaby learns the dead couple had been banned from the pub by Paddy, against Claire's wishes.


Barnaby tries to find out who might have borrowed money from the Lamberts. Carolyn was an avid smoker and everyone knew she lit up while on the phone. Did a phone call cause a spark which triggered the explosion - and was it deliberate?


Freddie returns to the Foundation and tells Dominic she has identified an imposter. Dominic calls a meeting and announces that Blaze is an investigative journalist, then Blaze claims that Jones is a private detective. Suddenly Lucy appears - with the news that Freddie is dead. They were lovers and had had a row.


Freddie was counting money and Ruth admits she knew about a hidden safe containing money, gold and documents. But when the police uncover existence of a promissory note they find the man who witnessed it died the same night as the Lamberts.


Can Barnaby and Jones find out who borrowed money from the Lamberts - and who would kill to stop them calling in the loan? The trail leads them to the pub, where they walk into another potential gas explosion...