Attorney Malcolm Rhodes (Lyle Bettger) invites McGarrett to sit in on a reading of a will written by the deceased contemporary artist Addison Barlow. He does so because he believes the document is an invitation to murder. In it, Addison -- who despised all of his heirs -- says that his estate will go to whoever is still alive after one year. When the will is read, Addison's weak-willed sister Laura promptly topples over, dead from a poisoned pill mixed in with her aspirin.
McGarrett arranges to have the body of Addison Barlow and his recently deceased niece, Carolyn, exhumed. Surprise! Both had been given drugs that led to their untimely deaths. On top of that, McGarrett has noticed a painting that exactly matches one owned by a collector friend. A check on it (and the others hanging on the Barlow wall) reveals that all of them are fakes. The originals have been sold by a gallery owned by Benilea (Sydney Lassick). Although he has bills of sale signed by Addison Barlow, the signatures have been forged. It turns out that the paintings were done by Carolyn as part of a scheme dreamed up by Laura's sponging husband David Thorpe (Anthony Caruso) and carried out by Addison's gambler son Lawrence (Francis Kamahele).
Soon Lawrence dies when the car he borrowed from his sister Eugenie (Christina Kokubo) goes off a cliff. The car has been tampered with. But Eugenie must have been the intended victim, because in Lawrence's pocket are found a container of allergy pills, some of which are laced with poison. That's how he was supposed to die.
Who will be next? Worthless brother Thomas (Eduard Franz)? Cold wife Riah (Helen Funai)? Daughter Eugenie? Son-in-law David? Swarming over the Barlow house, Five-0 discovers poison in Thomas's wine and on Riah's sewing needles. That leaves only Eugenie and David as the likely murderers. But it can't have been Eugenie. She hasn't even been in the house in a year.
Attention focuses on David as the likely villain. That is until it becomes clear that he couldn't have tampered with Eugenie's car. Then the penny drops for McGarrett. The real murderer is that vengeful old man Addison Barlow. He planned it all, even his own death, to get rid of the people he hated -- and to leave David holding the bag.
NOTE: This was a rare Hawaii Five-0 episode (the only one?) in which the puzzle wasn't solved until the last minute. Usually the guilty party or parties are known by us viewers, often from the opening scene.