This is one of the later Target novelisations - published in 1988 - and I suspect that is because this is one of the harder asks: two episodes exclusively set in the TARDIS with just the core cast; the looming doom not coming from some obvious foe but at first the crews scepticism of each other and then attempting to figure out the problem with the TARDIS; and not having a major climax (well, unless you count the fear before they find the BROKEN SPRING in the fast return switch. Nigel Robinson did a very good job at taking this story and adding additional elements to it to make it feel like a fully featured novel.

The main dynamic in this story is the relationships between the main characters - Susan and the Doctor massively distrusting the main two characters as they are most affected by the explosion; which forces Ian and Barbara to try and prove themselves right resulting in the solution coming together. The advantage of the novel format means that we can get inside the head of the characters which helps to deepen the story - much of the expansion in this story is this extra exposition around the thinking of the main characters. Robinson also adds a few additional scenes to this version - an expansion of the Doctor and Ian trying to find the fault with a trip to the TARDIS engine rooms which helps to add intrigue (especially the dynamic between the Doctor and Ian with the former increasingly distrusting the latter); and a rather silly scene with Barbara looking for a book to get some bedtime reading and stumbling into the Doctor's laboratory where the books all fall off the shelves to prevent Barbara from entering into a very dangerous room in the lab. These are more about scene building around the key scenes from the TV show (both happening before the Doctor tries to throw Ian and Barbara off the ship); and I think the Ian scene is more effective in that and has a greater fit.

Its also interesting to see the minor changes in the novelisation that are a side-product of this being a late publication. There are a number of minor wording changes to reflect updated Doctor Who terminology (the Time Vortex is mentioned a fair amount in this as an example - and the way that the scale of the TARDIS is talked about is very different from 1964); and the post-climax conversation between the Doctor and Barbara is expanded to deepen the fact that it marks a somewhat symbolic moment in the relationship between the Doctor and his first human companions - this being the moment where they rally trust each other.

However the TV story is better than the novelisation in my eyes - while Robinson did a good job expanding the story into a good book; its still at the end of the day a two parter with limited scope for expansion; and elements of this feel padded for the sake of meeting a page count. I think its worth a look over if you like the TV story and its one of the books with substantive changes from the TV story; but not a must read.