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The island of Barra in the Outer Hebrides may only measure 23 square miles, but its £19 million-a-year economic output belies its size. A huge part of the money-making machine is the island's shellfish industry, a multimillion-pound industry whose entire supply chain relies on CalMac ferries.

We meet Christina, Donald and their team at large-scale producers and distributors Barratlantic, on a day when an order of scallops worth £28,000, and due in France in 48 hours, is threatened by a last-minute ferry cancellation. At the other end of the scale, we spend time with oyster farmer Gerry as he harvests his oysters and delivers them for distribution to Barratlantic, before jumping on the Isle of Lewis to Oban for a prestigious oyster competition.

Fergus Barlow is a Merchant Navy veteran and one of CalMac’s newest recruits. A proud Mull islander and Tobermory man, Fergus is an engineer on the MV Loch Frisa, itself a new recruit to the CalMac fleet. We meet him as he directs vehicles and passengers onto the ferry deck, distributing the weight and height evenly to keep the ship balanced. Having spent many years on ships far from home, he has found his happy place on the route between Oban on the mainland and Craignure on Mull.

South of Oban, on the Kennacraig to Islay route, is the 38-year-old MV Hebridean Isles, captained by none other than Roddy ‘the Rottweiler’. One of the oldest ships in the fleet, she is being taken out of service for essential repairs to her engines, which have been gradually losing power. With overhaul season just around the corner, she is needed to cover a freight run by one of the biggest and most powerful ships in the fleet, but currently her engines won’t cut the stormy seas of the Minch. The repair job takes longer than expected, resulting in further cancellations to trips to the island. With mystery turbo thruster issues still blighting the ship, the chief engineer and his team work tirelessly to locate the fault and fix it, while queues of passengers begin to grow at the port.

After a weekend of weather disruptions and technical faults on numerous major vessels, area operations manager Fiona Galbraith is juggling displaced people and provisions while working out how to keep the supply chain running. From her port office at Lochboisdale, Fiona fields calls and emails from headquarters and worried captains.