There's a monster in the rural Scottish village of Lochanfoy, and it's not Nessie. At least one person in this tight-knit community is a brutal and sadistic killer.

The locals are in shock after the murder of local piano tutor Niall Swift, whose brain was gruesomely removed through his nose before he died. A human heart has also been found on the shore, while a body lies still undiscovered at the bottom of the loch. Oh, and now part of Niall's brain has turned up in a ziplock bag. 

The first couple of episodes set up three suspicious characters. Here is the evidence against each:

Craig Petrie

Craig Petrie, played by Alastair Mackenzie, is the head of Lochanfoy college.

The evidence against him:

– He was the one who discovered the body, while apparently on a jog in his lunch hour. But we know that he took Niall's phone after finding his corpse and later chucked it as far out into the Loch as he could manage. A witness said she saw him put something into his pocket, but Craig lied and told police it was his own phone – which conveniently wasn't working.

– He keeps his home office locked and is suspiciously antagonistic towards his wife when she mentions the murder.

Leighton Thomas

Ex-offender Leighton Thomas (William Ash) killed two men 20 years ago and served his time, entering prison almost illiterate and emerging with a degree. He runs a local boat tour business and few local people know about his past.

The evidence against him:

– Considering Leighton's past, he's a suspect by default. There is no real evidence against him at present and he has an alibi for the time of Niall Swift's murder.

Dr Simon Marr

Dr Simon Marr (John Heffernan) is the local doctor, a recent widower and a father of one.

The evidence against him: 

– Niall Swift was at Dr Marr's house to give a piano lesson to his daughter shortly before the murder. But Christian Dr Marr fired him in a heated conversation that suggested he was homophobic towards Niall. He handed him a CD that he'd snapped in half and told him not to come back. Why?

– During this conversation Niall told Dr Marr: "You're a hypocrite. I taught Bethan McGrelish." This seemed to put the fear of god into the village doctor – what secret does he have to hide, and is this a motive for murder?

– He gets very shirty with DS Annie Redford when she hears about the argument and starts asking questions, refusing to cooperate because she's not technically working on the case.

– Teenager Evie thinks Dr Marr could be the culprit because he once molested her during a medical exam.

– He drops a package off at the abattoir, though this turns out to be a dead wolf.

The Loch airs on Sundays at 9pm on ITV