In the short story Problem at Sea, Ellie Henderson was a passenger on board a Mediterranean cruise ship together with Hercule Poirot. A single woman travelling alone, she was described as someone with a slim well-preserved figure with keen dark eyes and grey hair. She was "a woman of forty-five who was content to look her age." Poirot was particularly impressed by the diplomatic and adept manner with which she handled the sarcastic remarks made by General Forbes about John Clapperton.
Ellie took pity on John Clapperton and the way which he was being bullied by his wife Adeline Clapperton. She admired him, commenting that he looked "soldierly". Unfortunately for her, his attention was often occupied with two younger girls on board the ship, Pamela Cregan and Kitty Mooney. Ellie continued to have sympathy for Clapperton even after she knew that he had murdered his wife. She told Poirot that the manner in which he had exposed Clapperton was a "cruel trick".
Ellie Henderson was played by Ann Firbank in the film adaptation of the story which was episode 7 of Series 1 of ITV's Agatha Christie's Poirot drama series. Her portrayal was very faithful to the original story.
There was one added sideplot involving Ellie. She had purchased a string of amber beads from a bead seller. A string of amber beads was later found in the cabin of the murdered Adeline Clapperton. This made Ellie a suspect for a time, especially when she told Hastings that she appeared to have lost the beads she had bought. Poirot thought it a cruel irony that when Clapperton had planted the beads to divert suspicion to the itinerant bead sellers, he had ended up casting suspicion on a person who truly cared for him.