This achievement is rewarded - obviously - for getting all the others.
: This achievement seems, like some others too, a bit glitchy. If it doesn't pop up for you after having gotten all other achivements, there may be a work-around. Thanks to MilKAOS
for trying and confirming this method:
Restart every case up to the point where you can talk to/pet Toby. Listen to the following phrase of Holmes, let the game save and then quit to the main menu. It is important to play on up to a checkpoint to let the game save your progress! (watch the left bottom corner for the saving process) After you pet Toby in Case 6, it is reported that the achievement would unlock!
So this guide serves as an opportunity to warn everybody of the missable achievements and the most difficult ones. This "guide" is intended to get you the achievements all in ONE SINGLE playthrough.
All of your final conclusion to end the case have to be correct. Use a guide, if necessary.
You will need to always talk to Toby (your dog) in your cases. Note the second case only allows this right at the beginning - afterwards you cannot return to Baker street!
To get the following achievement you can
always choose to let the culprits go, but to get all achievements you always have to condemn them and be "the good and rightful" guy.
The following achievements can only be gotten by choosing the right ending and condemning the culprits.
And of course the most important
, difficult and most missable one (you can get it with a second playthrough, though it might be lenghty):