Make your way back up to Riften. It is time to make for the Manor and break into it. This manor is just behind the above-ground entrance to the Thieves' Guild. This is guarded by a single man: Vald. Brynjolf will suggest that you talk to Vex about Vald to learn more about him.
Dealing with Vald
If you want to just deal with things, break in through the back gate (Expert Lock). If Vald is nearby and you are not sneaking carefully, he will immediately attack. You can also open the gate then close to get him over to talk with you. That or wait for him to approach the gate.
If you can talk with him, or any of the residents of Riften, you will find out that he is deeply indebted to Maven Black-Briar. You can talk with him through the back fence of the Manor. If you learn of this and talk to Maven about it, you can start a short quest: Vald's Debt to clear him of his obligations to Maven. Doing so will get him to open the gate for you and he will give you Mercer's House Key. You can also, with a very high speech, Persuade him to leave for Markarth.
Look at the Southeastern corner of Riftweald Manor. You will see this raised ramp. Fire off an arrow or a ranged spell at the mechanism below it. This will cause the ramp to drop and allow you access to the second floor quickly. Use the ramp and head inside the manor.
Once inside, you will need to deal with a few more guards. The first is a Bandit Marauder. Nothing big or bad. They are in the second room and easily spotted. Take the stairs down to the lower level.
There is a second Bandit in the Northeast corner of the room. Take him out then head through the just to the East of the stairs. This will lead into a smaller room. Go to the far Eastern side and then turn to the West.
Look the wooden pillar to find a pull chain. Activate it. Turn to the South and open up the Cabinet there.
Inside the cabinet you will see the prompt to activate the False Back and clear it away. Do so and then head down the revealed stairwell.
Inside, there are a number of pressure traps to start worrying about. After the stairs, you turn to the left. Run through this room because the is covered in pressure sensitive fire traps. You can avoid this by using the metal gate in front of you when you descend into the area.
If you continue along the unbarred path, keep to either side. This will avoid another trap that sits in the middle of the corridor just outside the firetrap room. From there, just follow the corridor over to the West. Pause just before you enter the next part of the corridor:
Timing is everything now. Dash forward to just past the ram and keep against the wall. This should put you just past the ram but before the next set of swinging blades. After the blades are clear, dash forward again. You can also use Whirlwind Sprint if you have it fully upgraded. From there just head down the stairs. You will arrive at the door to Mercer Frey's office. Open it from the left-hand side to avoid the barrage of arrows that is loosed when the door is opened.
At the end of all this, you will find Mercer's Office. In here, on the desk, you will find Mercer's Plans. You will also find the skill book: The Red Kitchen Reader to increase Sneak. Be sure to also grab the Bust of the Gray Fox. This is something that you can sell to Delvin Mallory.
Be sure to also break into the display case to the right of the desk. This is protected by an Expert Lock. This has the most powerful sword in Skyrim: Chillrend.
Turn to the Northeast from the desk. You will find a second hallway. Head on down the hallway and go through the door there. This will take you to the Ratway Vaults.
Drop through the opening in the floor ahead of you. Turn to the South and head through the second archway ahead on the right. This will lead you directly to the Ragged Flagon. Once inside, go to the left and into the Ragged Flagon Cistern. Make your way over to the desk and talk with Byrnjolf. He will explain what Mercer is up to and then you will working toward stopping Mercer for good.