Poirot and Hastings receive a message with a prospective job offer from Emily Arundell. Ms. Arundell has recently survived an attempt on her life, and suspecting a member of her family, changed her will in favor of a servant, while hiring Poirot. By the time they arrive, Ms. Arudnell has died, apparently from illness, causing the detectives to embark on a quest to find out whether it was murder, and who is responsible.
Dumb Witness is also notable as the last novel before Curtain to have Hastings as a narrator.
- Exact Words: As usual, Poirot does this, telling Hastings he suspects there will be another murder, but letting him draw his own, wrong conclusions about what he means.
- Gossipy Hens: Ms. Arundell's friend Ms. Peabody can get into this, although she's also good at keeping secrets that she views as more important.
- Granola Girl: Isabel and Julia Tripp are proto-examples: aside from spiritualists, they are also vegetarians, theosophists, British Israelitesnote and Christian Scientists.
- Happy Marriage Charade: Poirot comes to feel that Bella Tanois is far less devoted to her husband than he lets on, and feels that she had to settle for marrying him.
- Heroes Love Dogs: Hastings ultimately adopts Emily's dog.
- Inheritance Murder: Both Poirot and Ms. Arundell feel this was the reason for the first attempt on her life, and possibly the second one as well, given how she didn't tell everyone involved that she'd changed her will.
- It's for a Book: When speaking with some of the witnesses, Poirot claims he is writing a book about General John Arundell, Emily's father.
- Lovable Rogue: Arundell's nephew Charles gets this treatment, being a self-admitted wastrel, and somewhat shady figure, but one who people still find charming.
- Mailman vs. Dog: Discussed. Poirot has a theory about why dogs attack the mailman; they think that the mailman is an unwelcome guest, because he often comes to the door but never allowed in.
- MayDecember Romance: One of Ms. Arundell's sisters (Bella's mother) married a man about twice her age.
- Medication Tampering: The victims liver pills are doctored with phosphorus. The hint is given by the aura seen around the woman: the phosphorescence of her breath.
- Moment of Weakness: Ms. Lawson admits that when Ms. Arundell asked for her will shortly before dying, Ms. Lawson lied that it wasn't present due to believing that she intended to destroy it and knowing she inherited, something that has clearly caused her a great deal of distress.
- Old Maid: Ms. Arundell and two of her sisters died as elderly bachelorettes and Poirot suspects their niece Bella married simply to avoid this fate. Emily Arundell's best friend, Miss Peabody is another example.
- Shady Real Estate Agent: The local real estate agent is used by Poirot and Hastings as a source of information while they are supposed there house-hunting. When Hastings says that the man assured them Ms. Arundell's death was accidental, Poirot asks if Hastings also believes the man's claim about what the house they saw was really worth, causing Hastings to admit that Poirot has a point.
- Stalker with a Crush: Charles and Theresa Arundell's father followed the murder trial of an attractive woman, collecting clippings from it, then after she was acquitted, took a train to London and asked her to marry him.
- Uptight Loves Wild: London party girl Theresa and reserved, unemotional local doctor Rex Donaldson seem to be very different personalities, but have a secure and loving romance.
- What Does She See in Him?: Several people wonder why Theresa and Rex are together. Even Theresa can't really tell why she loves him.