On vacation in the Scottish Highlands to visit his Uncle Michael (with Sally and Mildred in tow; Enright comes over later), Mac walks onto the grounds of the McMillan castle and shouts for Uncle Michael. The response is a rifle shot. Mac rushes in and finds Uncle Michael's lawyer and his housekeeper, who are trying to break into the "keep" which Uncle Michael uses as an office. Uncle Michael is dead on the floor, shot through the head with a rifle lying beside him. All the evidence points to suicide, but Mac won't believe it -- especially when two men run away from the castle on consecutive nights. Mac finds that the McMillan family has a centuries-long feud with the neighboring MacCready family, dating back to when a McMillan soldier caught a MacCready hiding in a secret passage on the eve of a battle -- and walled him in alive. The lawyer and several others in the case are MacCreadys, and perhaps one of them decided to finish the feud. Uncle Michael's irresponsible grandson soon arrives, inheriting the castle and, it's rumored, a hidden treasure. The grandson is also running the Highland Games, which spark monster sightings in a nearby "loch" (lake) similar to "Nessie." While hunting, Mac ducks several rifle bullets, and Sally dodges a huge man who practices caber-tossing -- at her. It's clear that someone believes in the existence of the treasure and thus has a perfect motive to kill Uncle Michael -- but who? The mystery deepens when Enright and Sally find that a wall of the keep has been recently re-mortared and the corpse of the long-dead MacCready is inside his secret passage. Is it a clue to the locked-room mystery, or another red herring?

McMillian has to track down a satanic cult that has taken a deadly Interest in Sally.

A business magnate and friend of Mac's is getting mighty edgy when a car almost runs him down. Mac visits the businessman's upper-floor office, but outside the door hears glass shattering and a woman screaming out the businessman's name. Mac rushes to the broken window, looks out, and sees ... nothing. The woman, the businessman's secretary, won't talk. Mac leaves Enright in charge and goes to pick up Sally, hoping she can interview the woman. As they return to the building, they see a human body plunging down the facade of the building. This time, it really is the businessman. Did he disappear in mid-air for three hours, or did he attempt suicide, stop and then try again successfully -- or was he pushed out the broken window?

In the series' first two-hour episode, Mac is called back to his previous career in CIA intelligence when an agent is found fatally poisoned in a hotel room. Before dying, the agent had grabbed the Venetian blinds over his window and twisted them. Mac and the intelligence community realize the dead man was identifying his killer as "Venice," a notorious rogue agent who sells secrets anywhere and murders anyone who might recognize him. Mac and other agents seek out a woman named Elena Standish, an undercover operative who is the only person to survive a Venice attack. Venice goes after Elena several times, killing a contact and wounding Elena. Mac suspects that Venice is someone very close to him in the intelligence community.

Mac's college football team has a reunion coming up, but someone plans to spoil it with a gun. The killer, using a team photo as a reference, shoots the team members in the order of the numbers on their uniforms. Mac has to try to figure out the killer's motive before his number comes up -- literally.

Mac's incarcerated double is "sprung" from prison in a fake breakout so that he can impersonate Mac while Mac goes undercover to break a smuggling operation.