This is story related and cannot be missed. It is a short case that starts once Sherlock Holmes solves the fourth case.
Spoilers:*** Spoiler - click to reveal ***In Holmes's apartment, go to Kate's room. Exhaust all items you can select. Pressing 
will initiate an imagination sequence. Follow it by exiting the Kate's room to find yourself, not in the living room, but in a graveyard.
Follow the path to a locked door. Break the lock by following the QTE sequence--move the reticle inside the red circle with 
and press 
several times at each prompt.
Confront Alice and exhaust conversational choices. Resist the temptation to hit her--Holmes's weapon is his mind not the palm of his hand. (Hitting her, however, will not impact the outcome). Create a character portrait of Alice afterwards. You note that items are missing. Use imagination to enter your past recollection of the room and select the five missing items--gramophone, grimoire, candles, circular board, & portrait. Go to the deduction screen to create family crypt.
Go back to the cemetery and into the family crypt. There are 8 writings on the floor--identify all 8. Also select the grimoire & the candles, the briefcase & Kate's diary, and the portrait.
Enter the small room further inside the crypt. There's a mummified corpse at the table. Select it, pick up the document. Check the bed. Go to the chest. Read the inscription. Select the map and the lamp. Leave and you are given a prompt to select four crypts.
You have enough clues to figure out that you need to select Shaw, Fitzclarence, Evans, and Irwin. Imagination mode then shows two green lines intersecting at the crypt you need to go to. On entering, go to the table. Select a star shaped object with an exclamation point, photo, candlestick, & notes. While leaving check the poem and ribbon on the floor as well as the inscription on the tomb. When you find you can' t leave, check the astronomical chart on the left wall. You can now pick up the star shaped object from the table.
When returning to the astronomical chart, you have to connect certain stars to correspond to the drawing shown in the lower right. Use it with the below guide to assist you. Left and right in the guide are from your perspective. So, when it says left foot, this is the foot to your left and is actually the figure's right foot. Your starting point is a star left of center on the belt. You return to this point four more times so it plays a central role in connecting the stars:
Belt (starting point), right skirt, left hem, belt (starting point), left of left skirt, right hem, left foot, right foot, left hem, right hem, belt (starting point), right shoulder, right of head, belt (starting point), left hand (cuff), left shoulder, belt (starting point), head, right shoulder, left shoulder
When connected properly, a door opens up under the tomb. Follow it outside. Meet up with Watson and take the boat to the ship. Save Kate in the QTE (move the reticle inside the red circle with and press ) There are several QTEs during the ensuing sequence. All of them are moving the reticle inside the red circle with and pressing except for one which is a single 
QTE followed shortly thereafter by a single 
QTE. Holmes will have various timed responses. If there is a choice between Alice or Kate, always select Kate. Remind her you are family, that she is a Holmes, and that she had a childhood as a Holmes. Depending on how you solved the cases, the dialog will differ. If you have condemned anyone, point out (to Kate) that they were criminals. At the end of the discussion there are two QTEs--the first on the rope above and the second on Kate. They are both the standard moving the reticle inside the red circle with and pressing .
A cut scene follows and the achievement will unlock as the credits begin to roll.
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