The final story of the Doctor's third incarnation, this book plays on arachnophobia long before the movie of the same name.
The Doctor is on Earth, following up on an area of interest to him (psychic abilities) when an unfortunate incident between public performer and crypto-psychic, Professor Clegg, and a blue crystal the Doctor brought back from Metebelis Three brings to his attention the interest of the giant spiders from Metebelis Three.
Did I say "spiders"? Excuse me, they prefer to be called Eight-legs - so excuse me please ladies.
And very creepy they are, too. There they are with psychic powers, and able to possess humans by crouching on their backs (and disappearing, so you don't know who's really possessed).
The story is also replete with Buddhist imagery, the return of the disgraced Mike Yates, a strong role for Sarah, a very real and comfortable friendship between the Doctor and the Brigadier, and a surprise person from the Doctor's past. It's a big story with a lot to say.
Terrance Dicks adapts this story with a lot of care and attention, making this a worthwhile read.