In the novel Murder in Mesopotamia, Eric Leidner is a Swedish archaeologist and the second husband of Louise Leidner. At the time of the events of the novel, they had been married less than two years. He is the head of the University of Pittstown Expedition in Iraq, and is interested in pottery.
Dr Leidner is described as a man of "middle height with slightly stooping shoulders, a brown beard and gentle, tired eyes". He has a "rather nervous, hesitating manner", and there is "something gentle and kindly and rather helpless about him".
One night, Louise hears a noise in the antika room, and alerts Nurse Leatheran and her husband. Louise reacts with fear for herself, but Dr Leidner is only concerned about the artifacts in the antika room, and goes to investigate.
On the afternoon of the murder, Dr Leidner is on the roof of the expedition house, working on the pottery. He comes down from the roof, saying that he has made a clearance on the roof, which should please Louise, because she had complained that there was not enough room to walk. He goes to her room to tell her, but finds her dead.
Dr Leidner collapses from the shock of finding his wife dead. Nurse Leatheran and David Emmott carry him to his room.
Poirot asks Dr Leidner why he sent for a nurse, not for the police or a private detective, when his wife received threatening letters. Dr Leidner admits that he thought Louise might have written the letters herself, as he had noticed the similarity of the handwriting, which Nurse Leatheran had also noticed.
When Poirot suggests that the letters were written by Louise's first husband, or his younger brother, and that this man was one of the expedition staff, Dr Leidner says that it is absurd.
Later it is revealed that Eric in reality is Frederick Bosner, Louise Leidner's first husband, who was thought to be dead for several years. Frederick and Louise had been married for only a few months, when she found out that he was actually a spy on German pay. She reported him to her father, who was in the War Department. She had initially believed that he had been shot as a spy. However, after receiving the first letter claiming to be from him, she went to her father, and found out that Frederick had escaped, but had been involved in a train wreck a few weeks later, and his dead body was found amongst others. There had been a certain amount of disfiguration, and so it was impossible to say for certain whether the body was truly Frederick or not.
Poirot explains that Frederick emerged from the train accident with a new identity, that of a young Swedish archaeologist named Eric Leidner. He still loved Louise, and was determined that she should belong to no other man. He kept himself informed of her movements, and despatched a letter whenever he felt it necessary. He imitated some of the peculiarities of her handwriting in case she took the letters to the police.
Frederick reentered Louise's life as Eric Leidner, and she did not recognise him. He convinced her to marry him. After they were married, he sent another threatening letter, in order to convince her that Frederick Bosner and Eric Leidner were two separate people. He also staged a gas poisoning attempt in their flat, for the same purpose.
After two years of marriage, he started sending the letters again, because he had discovered that Louise was having an affair with Richard Carey. He also started preparing the scene for murder.
On the afternoon of the murder, Dr Leidner was on the roof. He dangled a mask, with which he had previously scared his wife, over the parapet, so that it tapped on her window. When she opened the window and stuck her head out to see who was playing this trick on her, he dropped a heavy quern on her. He had previously passed a cord through the hole in the quern, and used that cord to haul it back onto the roof.
After an hour or more had passed, he went downstairs, and entered his wife's room. He moved the body to a position between the bed and the door. He noticed a small stain on the rug in front of the window, and switched it with the rug in front of the washstand. This was so that if the stain was noticed, it would be connected with the washstand, and not the window.
Poirot also explains that the hiring of Nurse Leatheran was also part of Dr Leidner's plan, because it was essential that a reliable professional witness would be able to state that Louise had been dead for over an hour when she was found, which would have meant she had been killed at a time when everyone would swear that her husband was on the roof. This was to prevent suspicion arising, that he might have killed her when he entered the room to discover the body.
Poirot further explains that Miss Johnson was killed because she had discovered that Louise had been killed from the roof, through the open window of her bedroom. Dr Leidner substituted a glass of acid for the glass of water beside her bed. He knew that there was a chance that she would be considered to have committed the first murder and then been overcome with remorse. To strengthen this idea, he put the quern under her bed.
Poirot concludes his explanations by saying that everything is explained, and it is psychologically perfect, but that there is no proof. However, Dr Leidner says that he will not deny truth. He also says that he thinks he is rather glad, and that he is tired.
Dr Leidner says that he is sorry about Miss Johnson, and that it was not him, it was fear, that made him kill her. He also comments that Poirot would have made a good archaeologist, as he has a gift for recreating the past.