Serial - most successful podcast ever - reaches finale

Adnan Syed has been in jail since 1999. Credit: Serial

An audio series about a 15-year-old murder mystery that has captivated global audiences reaches its conclusion today.

Serial, whose podcasts are available free online and which has been broadcast on Radio 4 Extra throughout December, will have its twelfth and final episode aired tonight at 9pm.

The globally popular show involves American journalist Sarah Koening re-investigating the true story of a teenager and the ex-boyfriend convicted of her murder on apparently thin evidence.

High-school student Hae Min Lee was murdered in Baltimore County in 1999. Hae's 17-year-old ex-boyfriend, Adnan Syed, was sentenced to spend the rest of his life in prison.

The case against him largely rested on the story of one witness - Jay, a friend of Syed - who testified that he helped to bury Hae's body. However Syed has always maintained his innocence.

Adnan, centre, with Hae, third left. Credit: Serial

Koenig takes listeners on a journey looking into the case for new evidence and the story behind what happened on the day of the crime with a new podcast episode released each week.

The series, originally produced by WBEZ Chicago as an online podcast, has enjoyed enormous success around the world since the website began releasing episodes in October.

It quickly became so popular that other outlets produced weekly podcasts about the Serial podcast.

Radio 4 Extra said the series has reportedly been downloaded and streamed more than 21 million times, topping download charts in countries ranging from the US to Germany to India.

Listen to the final episode here: