Part one

UNIT has been assigned to monitor the switch-on of a revolutionary new power system – they know from experience such things can be tricky.

Nearby, Osgood and Lieutenant Bishop investigate mysterious disappearances – and appearances of trans-temporal phenomena. Kate would like to ask the Director some questions, but she's proving strangely elusive...until there's a murder.

Part two

There's a dead body in the power station. River Song is the prime suspect. And Kate is most concerned by the identity of the victim.

Meanwhile, Sam and Jacqui chase Vikings, while Osgood finds herself out of time. As deadly predators focus their attention on Earth, it seems activating the power of River Song could spell the end of everything...