The TARDIS arrives in prehistoric times and a dinosaur manages to get aboard the TARDIS when the Doctor has his back turned!  But the Doctor's solution is ingenious - and worrying.  He simply travels into the future and the dinosaur 'disappears' because it becomes extinct as them move forward in time.  It's disappearance is depicted as it shrinking!  Quite why this effect doesn't seem to impact on Ian and Barbara isn't explained but let's hope the Doctor doesn't travel too far into their own futures!