After the introduction sequence, you'll play this first mission as Jacob (the next mission will be as Evie).
Your objective will be to locate and assassinate Rupert Ferris. This is a pretty straightforward mission, as there are no additional synchronization objectives in it. You follow your objectives through each area (there are several) and once you reach his office, assassinate him. After a short cutscene where Ferris talks to Jacob, you have to escape to/on the train, and will enter hand-to-hand combat as part of it. Enemies will divert the train, which will then run off a cliff. Once you platform your way down to the bottom, the sequence will end.
In this mission you should be working towards regular and cover assassination challenges, as well as headshots with your throwing knives. You can also work on ledge kills as well.
Once you complete the sequence, the achievement should pop.