The Innsmouth Case

Warning: I don't know why, but occasionally events in the game will be different (i.e. the options you get in your office as to what to do, when talking to Dahlia upon meeting her, or when you enter the pizza joint). Best thing I can say is if you don't get the options below restart and try again.

Choose these dialogue options, which will also gain you the Vegan, by the way achievement since they go hand in hand:

...to go home
Say "Who is it?"
...invite her in
...which is a little bold
Kiss her hand
Keep flirting
Pay her a compliment
...her eyes
Go on about her eyes
"What does your daughter look like?"
...throw up into the drawer
"Don't you think there's something... strange about your daughter?"
Persist
"I have no further questions"
Accept the case
...by bus
Continue searching on your own
Show your ticket
Just keep walking
Sit down with someone
Sit down anyway
Talk to the stranger next to you
Do small talk about veganism
Continue talking
...the front window
Ignore
Keep walking
RUN!
...the city centre
...scope out the surroundings
Joe's Pizza and Burger
Look at the menu
Order a vegan meal
Leave the snack bar
Continue to the arcades
...gives you a jolt of nostalgia
Look at the games
...the first-person shooters
BOOM!
...queue for BOOM!
...strategically
...by running circles around them
Look down
Talk to Tabitha
Show her the photo
Er...
"...wrong?"
Follow Evelyn
Talk to the girl
"Why does your mother think you've gone missing?"
"Are your parents rich?"
Enter with her
"Hello beautiful"
"Dinner sounds splendid"
Follow her
...follow her without delay
Take a seat
Let the wine breathe!
"...you look stunning tonight"
Mention your vegan lifestyle
Yes, dammit! (Vegan, by the way will unlock here)
Silence
Introduce yourself
Decline politely
"Love me?"
"I still don't quite understand"
...makes you laugh
Wriggle around a bit
Keep struggling!
Make light of the situation
Bargain
"...make it quick will you"
"What did I do wrong?"
Grab the dagger (achievement will unlock here)