DI Poole attends a university reunion, but one of the group is murdered with an ice pick.

The team investigates the apparent suicide of an elderly resident at a retirement home.

The Doctor and Clara save the frightened crew from a threat in an underwater base, but the Doctor questions the haunting.

An alien warlord sets a twisted plan in motion in order to ensure his survival on a remote Army outpost, and the Doctor may have to do something unthinkable.