Question: what was the last series of unforgotten about

title: Unforgotten
text: Sara 's long lost father turns up out of the blue with some long - buried secrets about her past . Colin is shocked to discover that the rape allegation made against him was concocted by David Walker , who paid the supposed victim £ 5,000 to fake the story . Ellen Price revisits the location where Walker and his friends used to hold ' parties ' where children were abused . Tessa 's denial of David 's activities is called into question when Ellen recalls David 's wife having turned up at one of the parties , which forced a row in front of many of the guests . As Marion 's home life continues to spiral , she tries to make things up with Zoe . Simon makes the heartbreaking decision to cease the adoption unless Colin can convince Tyler to back off . Sara 's husband uncovers evidence which confirms she was in Naples at the time of the murder . With all of the possible suspects now carrying a solid alibi , Cassie and Sunny are stumped as to who the killer might be . Sunny tries to reveal his true feelings for Cassie .