Simon Gray's cold war comedy thriller is based loosely on real events surrounding double agent George Blake who was imprisoned for 42 years but escaped with the help of a petty criminal Sean Bourke whom he met at Wormwood Scrubs.
The first two acts focus on George (Geoffrey Streatfeild) and Sean (Emmet Byrne) meeting, striking up a sort of friendship and then the immediate aftermath of the escape.
Gray chooses to focus more on the relationship between the two men rather which left me curious about the breakout.
But it also left me curious about their relationship as I wasn't wholly convinced why Sean was helping George.
Where it starts to get interesting is in the final act before the interval when the action moves to Moscow where George is living as a guest of the KGB and writing his memoirs.
He invites Sean to visit and it is here that the qualities that made George not only a good spy but a double agent are revealed. The friends' situation is reversed as Sean is 'persuaded' by the KGB that it would be safer for him to extend his stay.
Through the second half Geoffrey Streatfeild keeps you guessing as to George's real intentions and just how involved he is in Sean's situation.
He convinces as a genuine friend but while there is nothing glaringly obvious in his performance there is the occasional hint of something more calculated in the character which starts to make you question everything about him.
The tension grows and Emmet Byrne does a great job of portraying a man moving from resigned then frustrated to worried as he slowly realises just how far out of his depth he really is.
If you leave at the interval you miss the nail-biting bits and seeing how George and Sean's friendship is turned on it's head. The first two acts feel like an overly long preamble but stick with it.
It's two hours and 20 minutes including and interval and I'm giving it four stars. Cell Mates is at Hampstead Theatre until 20 January.