The Lost Hero is the thirteenth and final episode of Series 5.
Previous Episode: Day of the Jacket
Next Episode: None
While trying to stop SKUL stealing a valuable data disc, Oscar behaves recklessly and is ordered to stand down by the Head of M.I.9, who tells him to stay off duty until he has calmed down. Angry and frustrated, Oscar decides to infiltrate SKUL and bring them down from the inside, on his own. He tells Rose and Carrie to keep quiet about his plan and they reluctantly agree.
When M.I.9 trace the SKUL agents to a warehouse, Oscar gets there first and persuades their leader Vincent to let him join them. With M.I.9 approaching, Oscar and Vincent escape. In the process, Oscar drops the signet ring his father, former M.I.9 super spy Edward Dixon Halliday, gave him before he disappeared 11 years ago. When Rose and Carrie arrive, they spot the ring and realise that Oscar has found SKUL.
When The Grand Master sees Oscar, he tells him to prove his loyalty to SKUL by helping him to kidnap the Head of M.I.9. Meanwhile, Rose and Carrie realise that Oscar is in danger and decide to visit his foster family in the hope of discovering more about his plan. They are shocked to discover that he lives with an M.I.9 agent, not a family, and are more surprised still when they found out that Oscar has been desperately trying to find out what happened to his father. All the evidence suggests that he is dead, but Rose and Carrie spot a lead that Oscar's missed and set out in search of Edward.
- This is the last appearance of Rose, Oscar, Carrie, Donovan, Davina, Scoop and Avril. Mrs King isn't present in Series 6, but returns in Series 7.
- Jade Dixon Halliday is mentioned.