My PCs are on a (rather large) Rogue Trader vessel having just lost the trail of a murderer, thief, scoundrel, and trafficker in xeno-artefacts. I think I'd like their next mission to occur while in transit.

What I have so far is this:

They're either in their cabin or walking the halls when they hear a piercing shriek. Upon finding the source, they will either see a horribly mutilated body, only identifiable by a piece of jewelry, or somebody stabbed to death, the appearant murderer in the room, sobbing, inconsoleable, and incomprehensible.

For case 1: The murderer is on the ship, but how do they find him/her/it?

For case 2: The suspect did in fact kill the victim, but they have no memory of the act and claim to have felt the presence of someone, or something else just before the crime happened.

So here's the tough part, I've never written something like this before and need some hints, tips, and advice. I'd like to keep the combat to a minimum and focus on roleplaying. Also, the group consists of the following: an imperial world arbitrator, feral world assassin, feral world guardsman, noble scum, mind-cleansed techpriest, , void born psyker, and another psyker whose origin escapes me right now.

Thanks for the help!

I ran an adventure years ago that was a murder mystery using Call of Cthulhu it was set on a cruise ship. The actual killer was one of the PCs, he knew in advance, but played it perfectly.

How did you run it? What did you do to give the players leads, leave clues, and generally guide the session towards its culmination?

Heres a thought- a possessing warp entity is doing the killing. Possible clues-

On Psyniscience rolls the psykers can detect the presence of malign evil at the kill-sites. Possibly they can even detect the entity if its close

The enginseer can analyse the ships machine spirit memories and realise there was a temporary near-instantaneous breach in the Gellar field just before the first murder. The Gellar fields are weakening with each murder as the psychic energy of the death feeds the warp entity.

The entity is jumping bodies- until the players can round up everyone together, they cannot trust anyone.

if you have a priest, then his exorcisms, holy water and preaching can harm or discomfort the entity.

SJE

I think that the problem is that it is no mystery to the characters (or players for that matter) that entities of the Warp are somehow involved in this setting.

Why not lead them to believe just that? That way, when the truth is actually so much more mundane, their heads will spin?

Like a homicidal maniac who thinks that voices are telling him to do it?

Or, a bit more original, the killer is a sociopath (an expert deceiver) and sets it up so that another passenger appears guilty.

Or Xenos brain worms like in Wrath of Khan?

As for writing a mystery, I heard someone once say that the best thing to do is to work backwards: figure out the crime and what clues were left behind. Mix in a few false leads, like an NPC suspect or two who are caught lying about where they were or what they were doing during the night in question: it will turn out that they are innocent but are covering up for an unrelated crime or sinful act.

Thanks for the advice, Necrozius . Those are some really good ideas there. Especially the warp entity "red herring" with the actual culprit being quite mundane.

How do you handle clues mechanically? Do you make players roll to find, then roll to interpret or just tell them what they find and then roll to interpret?

In addition, make sure you have a couple of well designed npc that are suspects and have a reason to behave very strange .

Taking the "ship-in-flight" background

- travelling schola who is in fact a heretic (or heretek?). He get´s nervous if he gets any attention since he has prove of his blasphemie with him
(scrolls, aritfacts, marks on his body, whatever) but he is NOT the killer. He is on the trip to get to a special place on a special planet to
conduct/take part in a certain ritual.

-a shady looking guy which travels with this ship every 5 years (or what ever appears to be "regular by comparison") which was seen
sneaking around in the ship without a guide and already got caught in places he was not supposed to be
He is a smuggler who makes some money with sneaking drugs on board and selling it to crew members.

- a young man/woman who has fallen in love during the trip. His/her love interested is a crew member in liason (spoose?) with one of the ships officers.
Been out of his quarters to meet his/her love the night the murder happens... and won´t be in his quarter, so! Will keep his mouth shut, since the "board rules" are very harsh on such a matter ...

- a covered assasine. The ship in question is part of a "trade war" between to navigator houses, and the assasine is on board to sabotage the ship and/or kill some important crew members...in the hour between "ship out of warp" and "passengers leave". He will not draw to much suspicion....which will make him a suspect of the players. If they search his quarters, they mind find hidden assasine weapons (blades, poison, needler pistole, etc.) Hard to explain, huh?

- a criminal with a bounty on his head. He is a "marriage swindler" posing as wealthy merchant right now. On the "planet of departure" he swindled to be minor noble and cheated a noble lady out of her maidenhood. Her father wants him back. Alive.
- Of course, at least one head-hunter which is after the swindler is on board, too.

Try to let the players sort out the clues. Reducing the mystery to a series of Inquiry and awareness rolls is no fun for anyone.

Make sure that you have a backup plan for if they fail to spot the piece of cloth clutched in the dead mans hand.

Have some Red Herrings, but not too many. Some players will attach more significance to small clues than you expect, and could derail the adventure by spending too much time investigating a dead end.

Don't be afraid to steal ideas from other people. File the serial numbers off any Agatha Christie novel and you will have the basis for a really good murder mystery. Replace some elements with 40K level technology, and some of the npcs with those suggested by Gregorius and bobs your uncle.

Alex

I want to thank you guys for the good advice you've given, please keep it coming.

I think I have a good foundation to start writing a mystery. I'll try to keep you up to date.