A British doctor on Monday admitted trying to kill his mother’s long-term partner, who stood between him and an inheritance, by injecting the man with poison disguised as a COVID-19 vaccine.

Prosecutors say Thomas Kwan pretended to be a community nurse delivering booster shots and injected Patrick O’Hara with a toxic substance, likely a pesticide. O’Hara, 72, was given a toxin which caused a “rare and life-threatening flesh-eating disease,” Newcastle Crown Court heard, the BBC reported.

Kwan, 53, initially denied attempted murder but changed his plea to guilty after prosecutors laid out their case at Newcastle Crown Court in northeast England.

Prosecutor Thomas Makepeace told the court that Kwan was a “respected and experienced” family doctor based in Sunderland, about 15 miles from Newcastle. The lawyer said Kwan used his “encyclopedic knowledge” of poisons in his plot to kill O’Hara, who was “a potential impediment to Mr. Kwan inheriting his mother’s estate upon her death.”

Makepeace called it an “audacious” plan that was “stranger than fiction,” the BBC reported.

Makepeace said Kwan forged documentation, used a vehicle with fake license plates and disguised himself with head-to-toe protective clothing, tinted glasses and a surgical mask to visit the home in Newcastle that O’Hara shared with Kwan’s mother, Jenny Leung, in January.

Prosecutors said Kwan went to “extraordinary” lengths to disguise himself and prepare the attack at the victim’s Newcastle home, the BBC reported.

“As I suspect, would any of us, Mr. O’Hara fell for it hook, line and sinker,” the prosecutor said.

Kwan had also installed spyware on his mother’s computer so he could monitor the couple’s activities online and take pictures of them, the BBC reported.

The next day, in pain and with a blistered arm, O’Hara went to a hospital, where he was diagnosed with necrotizing fasciitis. Part of his arm was cut away to stop it spreading, and O’Hara spent several weeks in intensive care.

Kwan was identified with the help of surveillance camera footage. Police who searched his home found an array of chemicals, including arsenic and liquid mercury, as well as castor beans which can be used to make the chemical weapon ricin.

Police have not been able to confirm what substance was used.

Christopher Atkinson of the Crown Prosecution Service said Kwan had refused to identify the poison, “allowing the victim’s health to further deteriorate.”

“While the attempt on his victim’s life was thankfully unsuccessful, the effects were still catastrophic,” he said.

Kwan will be sentenced later.

“Thomas Kwan’s actions were utterly despicable,” Detective Chief Inspector Jason Henry said in a statement. “He used his experience as a doctor to deceive the victim into thinking the medical appointment he had arranged was genuine before administering the poison which has caused him unimaginable pain and suffering.”