Image: UbisoftOfficial screenshot from Ubisoft.
A Simple Plan is the story achievement for completing Memory Sequence 2 of Assassin's Creed Syndicate. It unlocks when the twins finish their first job in London, a single-memory sequence that sets up the gang war against the Blighters.
Play the main memories of Sequence 2 through to the end. Optional objectives do not matter for this one; only the 100% sync achievement, Mentor, asks for those.