An 80 year old woman with dementia walks into a police station to report the 60 year old murder of her husband, a London bobby. The official report says he fell into the river, but the team's investigation uncovers old secrets, After Steve's teenage son is arrested for buying weed, Steve encounters the ex-wife he hasn't seen in 10 years.

Traces of blood on a Roman sword are linked to an unidentified, headless corpse found in 2008, Taking up the unsolved case, the team enters the world of Roman battle re-enactors. Just as Danny starts having feelings for someone new, he learns that his wife might be discharged from the secure hospital where she has spent several years.