When Rhys's elderly Uncle Bryn has a heart attack while listening to the shipping forecast, it seems like another routine death at Ivyday Nursing Home. But when Rhys and Gwen go to collect the old man's effects, Gwen's suspicions are roused by another elderly resident. The conversation is cut short, though, by a fire alarm, one of many consequences of the mysterious power cuts that are sweeping the nation. Gwen has a hunch that something is wrong and her search leads her to Miss Carew, a suspiciously fit and strong octogenarian who, despite having supposedly terminal heart disease, has left Ivyday and gone back to work at the Computer firm she used to run. Miss Carew has been offered a deal by Fitzroy, a wandering alien with an aversion to electricity who is looking for a home. It's a deal that Miss Carew can't refuse. But the consequences for planet Earth are unthinkable.
Torchwood are chasing aliens down the M4, when Jack accidentally blows a hole in the Severn Bridge, and the SUV hits the water. Whilst submerged John, Gwen and Ianto hear a strange noise, which, back at the Hub they realise is a cry for help. They track the cry to its source which turns out to be the deepest part of the Ocean - the Mariana Trench. Ianto rings old Torchwood flame, Carlie Roberts, who's an expert in marine geology, and Jack pulls strings with the US government to get them all on board the USS Calvin, an Arleigh Burke class destroyer, which is heading for the Trench. From there they board the Octopus Rock, the only submarine built to withstand the pressure at that depth, and follow the signal. But when the Submarine crashes, the team are left at the mercy of a hungry alien.
The brewery have called 'time' and it's the last night at The House of the Dead - the most haunted pub in Wales. Barry the barman has invited renowned psychic, Mrs Wintergreen, to hold a special séance to mark the occasion, and there's a big crowd hoping for the chance of seeing their deceased loved ones for one last time. But when Jack arrives on the scene, he's determined to stop them. Ianto is puzzled by Jack's behaviour, and Gwen is suspicious. Why is Jack acting so strangely? Then the ghosts start arriving - and all hell breaks loose.