My friends and I have not played Dark Heresy in a while and we are all about to get back together to play again this Saturday. I have already thought about the first part and have it all figured out. I wanted to do an adventure with my players that required them to intercept a truck or vehicle that contains a deadly criminal (Ideally psychic). They want it because there Radical inquisitor wanted them too. Nuff said. And they have to transport the the criminal from A to B without stiring up some panic within a city or something. Maybe the criminal wants to escape the acolytes. Maybe he wants to help. Maybe he doesnt even know whats going on. Oh and I need a reason why the inquisitor wants him. Well I need all the ideas I can get! So Thanks. Please Help!

If your acolytes are supposed to be transporting a psyker against his will, how are they going to prevent him from lashing out at them with his psychic powers? Drugs? Are you going to give them a warded tank to wheel him around in?

I would say that the psyker could be restrained w/ something inhibiting his powers (hooked up to drug iv, or head covered in a monstrous psy-blocking metal hood; I feel like there was an item like this in some book somewhere), and depending how the group springs him from his armored transport it might get damaged enough to let him have access to some of his powers.

From there, they can either lie or charm their way into calming him down, letting them all escape together. If this works, let the psyker blow up some cop reinforcements, or do something powerful to show them just how strong he can be. If they're attempts to talk fail, have them fight him. He'll be outnumbered, but they'll have to subdue him unconscious and he'll have psyker powers.

Ultimately, the reason your inquisitor wants him is up to you and what kind of Radical schemes he ultimately has planned. Perhaps he wants to hook him up to some ancient machine which uses psykers like batteries. Maybe he believes psychics are the next step of evolution, and wants to train him to work for him, and w/ the acolytes in the future. Or maybe he just wants to turn the psyker back in and get brownie points w/ an inquisitor lord, while making whoever originally captured the guy look bad. Good luck!

So the scenario you want is "Psyker transported from A to B against his will after being kidnapped from a convoy".

My first idea ist turning the psyker from "Criminal" simply into a "wyld psyker", one not sanctioned. This explains nicely why he is kept instead of killed (Psyker MUST be given to the black ship). The Inquisitor in Question might simply want him because his is powerfull but untrained.

First Prepare for the Attack
After arriving, the pc need to covertly find out as much about the transport and those doing it as possible. Perhaps they even need to aquire heavy weapons (if they could not justify them on transport or they simply weren´t provided). Not that failure in the roles to aquire informations and/or heavy weapons might lead to someone tipping off the Arbites/Enforcers so the convoy will be better guarded

As soon as they know the most likely root, they need to find a good place of an ambush (the better chose the more boni to concealment and the better the line of sight). This could be handled with Scholastic Lore (Tactica Imperialis).

Second: Attack the Convoy and get the Psyker alive
This might involve the Adeptus Arbites (giving the Tithe is Imperial Law) or not, but it is a good idea to introduce one heavly modified Rhino as prison transport (transport area turned to holding area with psy-damping as in radicals handbook). A second vehicle (a razorback with heavy bolters?) could work as an escort vehicle. Add two guards in the holding cells and another six in the escort vehicle. Add another escort vehicle if the Arbites/Enforcers were tipped off. Arm all of them as heavy as you desire.
After a hard fight, they pc have the pskyer ...and hopefully have a drug or device handed to them to dampen his powers. If it is a a drug, you have a "time pressure" since the doses will run low after a time.

Third: Get away
Hopefully, the pc have a good plan for the getaway. The attack will have been reported back and more troops will be sended after them. After living the sight of the attack, they need to get away rather quick and will face a lot of new established control points. How to sneak a drug (or device-hampered) psyker through enemy territory to a hidden landing sight where a guncutter is waiting? Whoever guarded the psyker before, now it will Enforcers and Arbites looking for them. Some patrols of the PDF (only SOME, mind you!) might be dettached to search outside the towns as well (good training for "seek and destroy", anyway!)....and how long will it take till the searchers might find the guncutter?

A good radical reason that i can think of for an inquisitor wanting a rogue psyker is for a delightful weapon in the Radicals Handbook

basically its a pyskic cannon that uses psykers brains as a fuel source

perhaps he wants another brain for the weapon

The Inquisitor wants that particular psyker cause his rantings are actually prophetic insights into events yet to come or threats considered to be known only to a few, and the Inquisitor wants to know how this is possible or use such divination for his own ends ....