San Francisco antique dealer Raymond Toliver reads aloud from an old, yellowing confession written by 19th century prospector Joshua Skinner. It describes how Skinner murdered his mining partner Gully Watson during the gold rush of 1849, built an enormous house on the site of the gold strike, then hid Watson's share of the mother lode - a cache worth millions that has never been found to this day. "Where is the gold?" Stemple's letter asks. "The answer is in my journals." As Toliver ponders aloud how anyone could murder their partner for a cache of gold, his own partner, Dalton Padron, bludgeons him to death with a sewing machine.
A few days later, Monk is entreated by a very drunk Lieutenant Disher to look into the whirlwind nuptials of his unglamorous, 50-something mother, Maria, to a dark, mysterious and much younger antique dealer named Dalton Padron - whom Disher doesn't trust. For one thing, Padron is insisting on taking his new bride to a marriage therapy clinic that very weekend for their honeymoon. "Something's definitely wrong," slurs Disher. "My spidey sense is tingling."
Monk agrees to investigate. He visits Padron's store, where he learns that Padron's partner, Raymond Toliver, has recently gone missing. He then finds an application for a visa on Padron's desk for a trip he is planning to Ecuador - and notes that under 'marital status,' Padron has checked 'single.' What's going on? Sharona proposes an ingenious way of finding out.
Monk and Sharona soon arrive at the Waterford Institute for Marital Therapy posing as husband and wife. They meet the institute's no-nonsense director, Dr. Julie Waterford, and then meet the two other couples who are there: Dalton and Maria Padron, and thirtysomethings Jeff and Rachel Sweeney.
In between therapy sessions, "Mr. and Mrs. Monk" observe Padron closely, noting his inordinate interest in the thousand or so leather-bound journals lining the shelves of the Waterford library - journals which, like the rest of the Institute, once belonged to a millionaire gold prospector named Joshua Skinner. Padron's interest in the books is quickly explained when they lift an ancient yellowed letter from Padron's pocket, telling of how Skinner murdered his prospecting partner and hid his share of the gold somewhere on the premises. Now Monk and Sharona also know the mysterious clue, "Where is the gold? The answer is in my journals."
Trouble is, the journals seem far from holding the secret to a hidden fortune - in fact, they appear to contain nothing but gibberish, and make no sense whatsoever. Monk is confused: he now knows Padron is after the gold, and that Padron's marriage to Maria was a pretext to allow him to come here and examine the journals. But where is the gold? Without it, Monk will be unable to prove Maria's beloved husband is, literally, a gold digger - particularly after Skinner's letter of confession is snatched back by an increasingly suspicious Padron.
Padron later lures his tormentors into an old, abandoned mineshaft near the clinic and stages a cave-in with Monk and Sharona inside. After a miraculous rescue, Sharona and a nearly catatonic Monk meet the benign Sheriff Mathis, a sixty-ish, small town peacemaker who can find no evidence that Padron sabotaged the mine. Monk and Sharona return to Waterford, more determined than ever to unmask Padron as the liar and con man they know him to be.
Meanwhile, Disher has found Raymond Toliver's body hidden in a 19th century dresser in Padron's antique store. Disher now knows something that Monk, up until now, has merely suspected: his mom has married a killer. He phones his mother and tells her to get the hell away from Padron... but Padron is standing mere inches behind Maria and hears everything. The line suddenly goes dead, and a horrified Disher contacts Sheriff Mathis and then rushes to Waterford as the race for the gold continues.
Padron, who has now bound and gagged Maria, is chopping apart a grandfather clock he is convinced contains Skinner's treasure while Monk and Sharona groggily pour through still more of the journals. Finally, the sheer weight of one of the journals, which he notices while resting one atop a wobbly bookshelf, gives Monk the key to the mystery. Eureka!
Sheriff Mathis arrives, slaps the cuffs on Padron, and frees Maria. Meanwhile, Monk drops a few of the journal pages into the fireplace, and explains that the clue "Where is the gold? The answer is in my journals." wasn't meant to be cryptic at all: Skinner had, quite literally, hidden the gold IN all of the journals - or, more precisely, in the ink that covered the pages. Joshua Skinner had mixed the gold with black ink in a cauldron, then spent the next 15 years filling one thousand journals with "golden" prose. Monk points to the burning journal leafs: the paper has curled away, leaving behind brilliant rivulets of pure molten gold.
Inflamed by greed, Padron manages to escape. He steals Mathis's gun, and herds everybody (including the claustrophobic Monk) into a walk-in closet. He is just loading the last of Skinner's journals into his SUV when Disher comes roaring up with his gun drawn. He successfully frees the others, thanks Monk, and leads Padron away to prison.
Another golden caper cracked by that most platinum of detectives, Adrian Monk!