Tactical Thinking

Tactical Thinking is a Brotherhood of Steel main quest in Fallout 4.

Quick walkthrough

 * 1) Speak to Lancer Captain Kells.
 * 2) Enter Old North Church.
 * 3) Kill Doctor Carrington.
 * 4) Breach the door to the lower levels.
 * 5) Kill Glory.
 * 6) Kill Tinker Tom.
 * 7) Kill Deacon.
 * 8) Kill Desdemona.
 * 9) Reprogram or destroy P.A.M..
 * 10) Report to Kells.

Detailed walkthrough
If the player character is wanting to get kicked out of the Railroad, accept Kell’s forthcoming orders and this becomes permanent, so if the player character has any loose ends to tie up with the Railroad, do so before speaking to Kells. The Sole Survivor has been informed that Lancer Captain Kells wishes to speak with you on the command deck of the Prydwen. After agreeing to his orders, head north into the North End neighborhood of Boston, and enter the Old North Church. The player character has a priority kill orders list. It's time to check those priorities off.

Remove all threats in the initial church chamber. It's wise to eliminate all foes before continuing into the crypt. Fight down into the catacombs. Locate and activate the detonator and blow a hole in the wall. The remaining primary targets are now acquired.

Enter through the hole in the wall and into the Railroad HQ. Dispatch all Railroad threats from the theater of combat

Locate the P.A.M. mainframe terminal or the automaton herself. Use the P.A.M. decryption program holotape given to you and use it in the mainframe. Either destroy the robot or use the terminal to reprogram her. Then return and report to Kells.

There is disappointment if P.A.M. was deactivated rather than reprogrammed. If she was reprogrammed, she is later transported onto the Prydwen.

Bugs

 * Deacon may appear shortly after the player character enters Old North Church, and if one kills him before the quest tells to, it may not count towards the objective. Reloading to a save prior to entering Old North Church might fix this. Alternatively, speaking to Lancer Captain Kells once all other objectives are completed may still allow to turn the quest in.
 * Deacon may become unkillable, unselectable in VATS but still shooting back. Reloading to a prior save and killing him with the Mister Sandman perk before obtaining this quest from Lancer Captain Kells might resolve this issue.
 * Deacon may not be encountered inside Old North Church or the Railroad HQ. Instead his quest marker can be found at a random spot anywhere in the Commonwealth. Fast-travelling to a location near his position may only result in him being somewhere else far away upon arrival. Once having caught up with him he can be killed normally. This is more likely to happen if the player took Deacon's mission Tradecraft prior to taking Tactical Thinking.
 * Detonator may not appear, rendering the mission impossible to finish. By leaving Old North Church, fast travelling to another location, then fast travelling back Doctor Carrington will appear in the first room of the church. By killing him the detonator should re-appear allowing the player character to continue the quest. He can also be shot through a small patch of wall on the left edge of the unopened secret doorway. VATS will pick him up with a 0% hit chance when you've found it.