As Alicia Bridges investigates the beginnings of an epic deception, she finds it spans across continents. At the heart of the deception is an upside-down world of gang crime, and a series of dilemmas. What secrets should stay secret? How far would you go to catch a killer?

One man is in jail, convicted for murder. But are there other people behind bars who might be innocent, here or overseas? In Canada, a secret recording captures another man admitting to murder, and the same story begins again, but this time with a different ending.

In this case, Andy Rose is under suspicion for the murder of two German travellers, Andrea Scherpf and Bernd Göricke.

His lawyer Tania Chamberlain recounts her shock at the contents of a VHS tape that arrives at her office one day. Eventually, the issue ends up in the Canada's highest court, and one of the country's highest-profile litigators, Marie Heinen, gets involved.

Back in Melbourne, Mary Cook's daughters speak about what happened to their mother, and give their thoughts on everything that followed.

STATEMENT FROM VICTORIA POLICE:

Victoria Police stands behind the rigorous investigation which led to the conviction of Glen Weaven in relation to the 2008 death of Mary Lou Cook in Narre Warren. We consider this matter finalised and will not be commenting further.

Victoria Police does not comment on the specifics or application of covert methodology. Covert methodology is deployed in the investigation of the most serious and violent crimes, and it is incumbent on law enforcement to pursue strategies to seek justice for victims and their families. Community safety is at the forefront of all decision making.

Investigative strategies involving serious and organised crime whether covert or overt, are subject to significant planning and risk assessment processes. Victoria Police applies internal operating procedures to covert investigations, which include strict parameters and risk assessment processes including consideration of human rights. The deployments are also subject to formal reporting and oversight regimes to ensure they are employed appropriately and do not adversely impact future court proceedings or investigations.

Victoria Police monitors the use of covert strategies in other Australian jurisdictions and around the world, to ensure our methods remain contemporary and that they align with best practice. Victoria Police accepts that covert methods are and have been subject to public discussion and interest due to judicial proceedings and media reporting, however further publicity does not benefit current or future application of the strategies.

It is important to note that while the concepts of covert methods may be in the public realm at various times, the details pertaining to the practical application are not. All investigations whether covert or overt vary and the mechanisms employed change depending on the circumstances at the time.

Success or results in terms of covert strategies is difficult to quantify. The methodology is employed as an investigative strategy with the aim of seeking answers and to identify the person or persons responsible for serious criminal offending. The outcome may not result in a criminal conviction but that does not mean there is not a result.

STATEMENT FROM QUEENSLAND POLICE:

The Queensland Police Service is committed to high standards of behaviour, transparency and accountability.

Our purpose is to keep people, places and communities of Queensland safe through excellence in policing and community safety. To achieve this, investigators employ numerous lawful strategies for investigating and solving all crimes. These strategies can at times include covert methodologies to progress investigations, which under all circumstances should remain covert to protect their effectiveness.

As with every investigation, investigators continuously review current investigative strategies, techniques and emerging trends to ensure such methods do not impede the solving of a crime.