The first episode is set two years after the previous series when Sarah is called to investigate the stabbing of a teenage boy - the case brings her into contact with Lizzie, who is now balancing life as a detective and mother, and into conflict with DI Kieran Shaw who is running Operation Perseus, an undercover police task force set up to bring down South London drug kingpin Shakiel Oliver. Kieran is working with DC Steve Bradshaw who has gone undercover to infiltrate Shakiel's empire.
The focus of Sarah and Kieran's conflict is 15-year-old Ryan Kennedy who witnessed the fatal stabbing but is also a key part of Shakiel's inner circle - as Perseus heads to its shocking climax, Ryan will have a surprising role to play.

The Tower 2 opens with Lizzie returning to Farlow station. Her boss and former lover, DI Kieran Shaw, partners her with Arif Johar. Their first call is to a domestic violence complaint against Mark Brannon, who is accused of beating up his girlfriend, Georgina Teel and terrorising their young daughter Skye. 

The antagonistic relationship between the experienced Sarah Collins and the novice cop Lizzie Adama is at the heart of The Tower 2, as they are forced to work together for the first time and begin to get a sense of each other's unique strengths.  

In The Tower's breath-taking opening sequence, a veteran beat cop and a teenage girl fall to their deaths from a tower block in south-east London. Left alive on the roof are a five-year-old boy and rookie police officer Lizzie Adama. Within hours, Lizzie has disappeared, and Detective Sergeant Sarah Collins is charged with leading the investigation. She not only has to track down Lizzie before she comes to serious harm, but also to uncover the truth behind the grisly deaths. What chain of events took two police officers and two children to the roof of that tower block - why did two of them die? Why did two of them survive?