The Doctor, while trying to return Ian and Barbara to their own time, attempts new combinations on the TARDIS' controls. The TARDIS blacks out after an explosion. Susan and Barbara blame aliens while the Doctor thinks that Ian and Barbara were trying to sabotage the TARDIS. Tensions flare when finally they realize that all this is the result of the TARDIS trying to telepathically contact them in order to warn them that the Fast Return Switch is stuck and that they are going backwards in time to the very beginning.