Eliza follows a lead from her father's journal, the final entry he made, with a single message, Woolwich Prison 99, she finds only 98 rooms there, and a man laying in wait.
Very imaginative and very different, Cell 99 is unlikely any other episode, it is far from the conventional type of whodunnit or murder case, it's a paper chase mystery, with mild horror tones.
It's multi layered, it's like a jigsaw puzzle, with one curiosity following the next.
Incredibly atmospheric, first off it looks great, the Docklands and external shots of The Prison look fantastic, and as for the interiors, they got it spot on, the lighting was perfect.
Stuart Martin gets a chance to show off his fine physique, you wonder where such a busy officer would have the time to get to the gym. Good to see Eliza looking a little disheveled for a change.
Nick Dunning plays The Duke's boss well, he's bone idol and impossible.
The best yet.
9/10.
