Henry is sick, Shawn tells Gus. He called to cancel their weekend camping trip in the mountains and Shawn couldn't be more thrilled, until he catches his dad driving through town, the picture of health. Now Shawn has to know why he was ditched, and he and Gus follow Henry to the Monarch Lodge, where they discover Henry is a member. After Shawn perfectly repeats the secret handshake, they enter the lodge to find Henry in a room full of robed lodge members, surrounding a dead body on the floor. The brothers have called Henry in for help, but he insists he's retired both from the force and the lodge and calls the police. Gus, meanwhile, realizes he's in a room full of doctors, and gingerly steps around the body to start networking. Shawn tells Henry that the deceased, Arthur Holmstrom, was murdered, possibly by poison, and no one in the room seems to be upset about his death. Henry's friend Ted Archibald, who called them in, is worried they did something wrong during Arthur's Patriarch ceremony. He had a ceremonial hood on for twenty minutes, and is worried Arthur might have suffocated. Lassiter and Juliet arrive at the scene. It turns out Lassiter has always wanted to be a member but has been blackballed by his soon to be former father-in-law, Irving Parker. Now Lassiter is determined to solve the case quickly to impress Irving and the rest of the lodge members. Meanwhile, Gus has been busy locking down an appointment with Dr. Downs, who has a large practice and is a big purchaser of pharmaceuticals.
Back at the Psych office, Shawn and Gus are researching the lodge when Lassiter shows up and asks for their help. He knows they have an in, and he wants them to share any information they get. They agree to help Lassiter, who warns them to keep a low profile. Of course the next day, Shawn is inducted as a member of the lodge. Now Shawn and Gus are able to move about freely and they discover that Holmstrom was being inducted as the Highmost Patriarch of the lodge, skipping the three other people who ranked above him for the spot, including Irving Parker, who, in the wake of Holmstrom's death, is the new Highmost Patriarch. They also notice all the plaques from various charities that the lodge has received for generous contributions over the years. Shawn also sees the hood Holmstrom was wearing when he died, displayed in a glass case. It's over 100 years old. But as he turns to comment to Gus, Gus is back schmoozing Dr. Downs and a few of his colleagues. So Shawn checks in with Lassiter who gets a call from Juliet. She tells them Holmstrom's tox screen came back showing a toxin resembling the venom of a Brazilian vine snake, but there were no marks or puncture wounds on the body, just a slight skin irritation on his face.
Shawn and Gus go to visit their top suspect, the new Highmost Patriarch, Irving Parker at his Export/Import company. In the warehouse, Shawn notices shipping codes on crates from several Brazilian airports. Gus has appointments with a bunch of new clients, so Shawn is forced to tell Lassiter alone that all their evidence implicates Parker. Lassiter agrees to interview Parker but tells Shawn there is no way he's a killer and to keep looking. Shawn and Gus decide to take a different tack and investigate Holmstrom. They sneak into his house but as they are looking through his office, they hear someone else breaking in. They hide as the intruder rifles through Holmstrom's things and leaves. When they come out, they see the intruder uncovered a hidden safe but couldn't get in it. From the many pictures of Holmstrom's late wife throughout the office, they figure out the combination is her measurements and discover two lodge ledgers showing differing amounts donated to charity. It seems like Holmstrom was skimming funds from the lodge but Gus notices everything is written in doctor's shorthand and in doctor's handwriting. One of the doctors was stealing and cooking the books. Holmstrom found out about it and was blackmailing the culprit not only for money, but for the Highmost Patriarch position as well.
Back at the police station, Lassiter finally gets Parker to talk, telling him that he has no idea how Holmstrom got to be Highmost Patriarch but that he was okay with it because Arthur was a friend who had health problems which were getting worse. This would be his only shot at it. Plus, he hasn't been out of the country in years and so wouldn't have been able to bring back a snake from Brazil.
Shawn and Gus are visiting all the doctors' offices from the lodge to compare handwriting. They arrive at Dr. Downs' office and see pictures on the wall from the third world countries to which the doctor has ministered. As Gus distracts the receptionist, Shawn notices an article on powdered forms of medicine and sees a sample of Downs' handwriting. It's a match. They call Lassiter and Juliet to meet them at the lodge where Shawn explains that Downs killed Holmstrom with a powdered form of the snake venom that he put in the ceremonial hood, the venom being part of an anti-venom that Downs uses in his charitable work abroad. Shawn has solved the case but declares he never would have been able to without Lassiter's help, putting the detective in the good graces of Parker and the rest of the lodge members, rescinding his blackball status.