The Doctor, in an attempt to return Ian and Barbara to their own time, experiments with a new combination on the TARDIS console. The result is an explosion, and everyone blacks out. When they recover, Susan and Barbara believe that an alien force was influencing the TARDIS.
The Doctor accuses Ian and Barbara of sabotage, claiming that they were blackmailing him to return them to their own time. Even susan begins to suspect her former teachers.
They all realise that the effects were the TARDIS's defence mechanism warning them of impending disaster, which has also been causing the pshycological disturbances that they had all been experiencing.
The fast return switch that the Doctor had used on leaving Skaro had become jammed, and was leading the TARDIS towards Event One - the release of energy that created the Universe. The problem is a faulty spring, and once replaced the TARDIS begins to operate the same as before, leaving the Doctor with some apologies.