It's Humphrey's last day as detective inspector, but there's no time to dwell, as the whole team is eagerly awaiting the town's annual pirate festival. Danger, however, is ahoy when Geraldine Desai's nautical exhibition goes up in smoke, leading to the ancient treasure map of pirate Elias Cutler going missing amid the chaos. On the trail of the missing map, Humphrey soon has suspicions when he discovers that Geraldine has been selling her memorabilia to pay for her husband's care and recently had the map revalued. Touching base with an antiques dealer, who is adamant the map is worth nothing, the team begin to question if they're dealing with a harmless prank, an insurance job, or something more sinister.

Meanwhile, Humphrey frets about a potential leaving party as Mr Smith prepares to take over. Smelling a rat, Anne recruits Martha and Kelby to launch an investigation of her own. Elsewhere, Esther braces herself to say goodbye to her daughter, unaware that Zoe has prepared a shock surprise.

Humphrey and Esther are left baffled when a dogwalker leads them to a man painted green and bound with ivy. Delving into the mystery of the mythical Green Man, the team discover an off-grid community living off the land. But when their woodland home comes under threat from a developer from the city, tensions rise and accusations fly as the image of the Green Man continues to crop up at crime scenes, including painted across a developer's office as it goes up in flames.

Learning more from the two warring brothers who own the land, with one siding with the off-gridders, and the other keen to get the diggers in, it's up to the Shipton Abbott team to uncover the ties that bind and those that don't. But when one alibi doesn't check out, it seems the battle lines in this extreme sibling rivalry aren't as clear-cut as they seem, as dark secrets threaten to be unearthed.

Meanwhile, the pressure on Humphrey from Mr Smith continues to mount as the deadline to lose a team member rapidly approaches. A family dinner looks set to offer some respite, but things go from bad to worse when it turns out Anne has invited Mr Smith as her special guest. Elsewhere, a determined Martha shocks all around her when she takes on some of Margo's advice, leaving even Humphrey and Anne speechless. And Kelby is joined by Esther and Margo as he anxiously awaits Agnieszka's arrival from Poland. But as the night draws on and Kelby continues to wait, Margo's theory that he's being scammed looks all but confirmed...

The team are baffled when an eyewitness, fisherman Jeff Simmons, swears that he saw a legendary mermaid on the night a seaweed farm was sabotaged. With no logical explanation for how someone could sabotage the farm and get away unseen amongst glowing bioluminescent waters, Esther braces herself for an excited Humphrey to run wild with the possibility that their prime suspect could be a mermaid... though it's clear that something else is playing on his mind.

Investigating the nets used to sink the seaweed farm, the only clue Humphrey has to go on is a mysterious tropical shell, along with the discovery that the farm is looking to expand. Could it be a protest from angered environmental groups? A personal vendetta? Or is a vengeful mermaid out to stop those who threaten her seas?

Meanwhile, Humphrey and Martha's new house renovations get off to a bad start when Humphrey's attempt at plumbing leads to disaster. Increasingly distracted, Humphrey comes clean to Martha about the weight on his shoulders as Mr Smith ups the pressure from the Hub for his decision. Martha and Esther finally find the time to let their hair down and throw caution to the wind when their night out ends in skinny dipping. And Kelby continues to gush about his new online girlfriend, though Margo reckons she sounds too good to be true.

When the team attend a Dark Morris Dancing event, the ‘Legend of the Knuckerhole Dragon', Margo finds herself at the centre of a crime when a member of her team collapses during their performance. With a severe allergic reaction to blame, and the victim's epi-pen case empty, Humphrey is stumped as to how the attack was triggered unseen whilst all eyes were on the stage.

As jealousy and secrets come to light, the targeting of the victim's allergic reaction begins to paint a sinister picture. And with a connection to the crime, Margo is put to the test as she goes undercover to see what clues a familiar face might hold.

Meanwhile, Humphrey and Martha take a big step on their homeowning journey, but Humphrey's mind is preoccupied when a powerful figure from the Hub arrives at the station looking for answers. Elsewhere, Esther is floored when Zoe suddenly reveals her decision to travel the world.

Humphrey and Esther are just as surprised as owners Clive and June when they arrive at the site of a burglary to discover the crime scene has been meticulously cleaned and returned to its prior state, despite reports that the room had been turned upside down. Clive's office is then ransacked before their house is broken into once again, leaving the couple at their wits' end.

Meanwhile, despite putting their fostering journey on pause, Humphrey and Martha find their decision back at the forefront of their minds as they await a message from someone from their past. Kelby grapples with Margo's insistence he keep up positive affirmations whilst dealing with a young teen held captive in a garden shed, and Esther wrestles with a tension headache from a cricked neck and stress. Zoe organises her a massage with a hot new masseuse in Shipton Abbott, but Esther struggles to relieve any tension when she discovers a connection between them.

Humphrey is beyond excited to be attending a book signing by his favourite crime novelist, Kenneth Linder. But when Kenneth is found dead the next day, Humphrey is baffled to discover that the details of his demise eerily mirror the murder in his final novel. But is it a coincidence or murder by design?

Meanwhile, Humphrey and Martha find themselves lost at sea when the Lily Bond is caught adrift. With their houseboat out of action, Humphrey and Martha soon find themselves back with Anne. Reluctant to live with her mother again, Martha is set on finding something more permanent, whilst Humphrey faces an impossible decision at the hub that will change his team for ever.