The creatures to be featured are the liopleurodon - a large, carnivorous marine reptile - and a raptor ancestor.
The sub is under attack by liopleurodon, knocking out the captain and leaving just one officer and the team to sail the sub to safety. The sub is sucked through the anomaly, knocking out the electrics.
Meanwhile, at the ARC Lester has to cope with the intervention of navy bigwigs, threatening to nuke the sub if the team can't fix the problem. The political fallout from any damage caused by the anomaly is too big to ignore. Jess and Lester are working against the clock as they attempt to distract the admiral long enough for the team to do their stuff.
While Connor heads off to fix the electrics Matt and Abby head out onto a mini-sub in an attempt to repair the external damage. But the constant threat of the liopleurodon is making their task even more difficult. Meanwhile, the freshly-woken raptor ancestor is stalking the corridors of the sub, having already taken out another guard.
Connor is still attempting to fix the internal damage when the creature attacks him. He struggles to fight back as he protects the ship from further damage. Matt and Abby turn up in the nick of time but not before a headlong collision between sub and liopleurodon allows the creature to escape and creates a massive leak, putting the team in further danger.
Racing against the clock (and the leak) Connor and Abby eventually manage to take the creature down with a combination of wet floors and an EMD bolt. Then Matt has a brainwave – perhaps they can use the torpedo to kill two birds with one stone – forcing the creature out of the sub and closing the anomaly in the process. Transferring it to the torpedo bay, Matt plans to flood the room and eject the creature as a distraction for the liopleurodon, allowing them to escape. Little do they know, though, that Lester is doing all he can to stop the admiral firing a nuclear torpedo straight at them . . .
The six-part series, which begins on ITV1 this coming Saturday at 6pm, was originally shown on digital channel Watch in May and June last year under an agreement that saw Series 4 broadcast first on ITV1 before being shown on Watch.
It stars Hannah Spearritt as Abby Maitland, Andrew-Lee Potts as Connor Temple, Ben Miller as James Lester, Ruth Kearney as Jess Parker, Ciaran McMenamin as Matt Anderson, and Ben Mansfield as Captain Hilary Becker.