The TARDIS's temporal grace defense feature, which supposedly prevents weapons from being fired within it, makes its only successful appearance here, preventing Eldrad from using her mental attack. In later episodes, its absence is glaring, most notably in "Earthshock". Eventually, the Doctor would admit that temporal grace was just "a clever lie." Then why does it work here? Precisely because it was a mental attack. The Doctor was able to convincingly bluff Eldrad into believing the attack wouldn't work, thereby instilling enough doubt to keep her from actually following through.
