Malcolm puts Oliver and Thea in a dangerous situation that lands them on Lian Yu with Oliver's former enemy Deathstroke. In a flashback, Oliver and Maseo return to Starling City to retrieve the Omega bio weapon.

Thea betrays Malcolm to the League, but Oliver sets out to rescue his lover's killer so that Thea won't suffer the guilt of arranging for a parent's death. In the past, Oliver and the Yamashiros are released from custody, but Amanda Waller has other plans.

Oliver has no choice but to accept Ra's's offer to resurrect Thea. In return, Oliver must become the Heir to the Demon.

Ra's tells Nyssa that she must marry Oliver or face death. Meanwhile, an unexpected ally travels to Starling City to ask Diggle, Felicity, and Laurel to help infiltrate Nanda Parbat. In the past, Maseo and Oliver try to find the cure for Akio before it's too late.

With time running out for Starling City, Oliver tries to stop Ra's from unleashing the bioweapon and killing millions.