We first see the mysterious Mr Smith in Rise, when he opens a package sent by the late Captain Montgomery containing case files relating to Johanna Beckett and John Raglan. As he puts in a call to the Capitol and then to Castle, we realise that now, after Montgomery's death, he has become the man to hold the evidence that has been keeping Beckett and Montgomery's family alive. He instructs Castle to look after Beckett and steer her away from investigating her mother's murder.
In Dial M for Mayor, Mr Smith reappears to warn Castle that the death they are investigating relates to Johanna Beckett's murder, and gives him a phone number. Castle eventually calls to meet him, and they rendezvous in an underground parking garage, where he confirms that the murder is indeed part of a plot to topple the mayor. In a second rendezvous after the case has been solved, Castle demands more answers, but Smith refuses to give them. All he will say is that he intervened to save the mayor because he guarantees Castle's presence with Beckett at work, and that "a well-placed pawn can be more powerful than a king".
At the end of Always, Cole Maddox, who has escaped after throwing Beckett off the roof of the hotel, tracks down Mr Smith and demands the evidence he holds on Maddox's employer. The following day, in After the Storm, we see Maddox torturing Smith, who refuses to talk or give up the information.
Castle and Beckett, working from a photo of Montgomery's, manage to identify him as Michael Smith, an attorney, and start a desperate search for him, fearing that Maddox has him. They find him alive but badly injured, and the file of evidence keeping Beckett safe burned in the fire. But Castle is convinced that Smith must have kept a second file hidden, and they hunt for it - only Maddox gets there first, and is killed when the booby-trapped file is destroyed.
Castle and Beckett return to hospital to see Smith. He explains he helped Montgomery because he owed him a favour, but now both he and Beckett are targets, and he advises her to hide. Castle and Beckett leave unsatisfied, change their minds and return, only to find that Smith has gone into cardiac arrest and died.
Two years later, with Beckett framed and on the run in Veritas, it emerges that a victim in the case has been seen with... Mr Smith. He had staged his own death to scare from the clutches of Bracken's men. He phones Castle and they meet again in the underground parking lot. He says there is one more piece of evidence – a recording of Bracken admitting to murder – somewhere, but no-one knows where, not even Bracken. He warns Beckett to run, saying Bracken is certain to kill her, and walks away.
In the season 8 episode PhDead, Beckett needs to find Smith once more; she asks Vikram Singh, now the 12th's tech, to "dig him up". Smith show up for a meet, continues to warn Beckett to leave him alone, and says he has a good life now - but he does give her a hint as to how to follow the trail she's after.
Mr Smith is played by Geoff Pierson.