Snowdonia. UNIT is running a training exercise with the Brigadier in attendance. Except it isn’t long before things start to go badly wrong. The fake aliens primed to attack the troops might not be so fake after all, and a temporal disturbance attracts the Doctor, Tegan and Turlough into the fray.

Old enemies are on the scene with a deadly plan - but they might be the least of the Doctor’s worries.

Because he’s recently been sent backwards and forwards through his own lifetime… and he’s finally going to find out why.