Her adoptive son Jack (played by Anthony Boyle) was blamed for her death and sent to prison, where he died while waiting for his alibi to be found.
However, in a shock turn of events Jack’s alibi, Dr Arthur Calgary (Luke Treadaway) arrives at the Argyll family home and reveals the young man was innocent.
The family chose to ignore his claims but many of those tuning into the show are desperate to find out who the real culprit it is.
And, it seems the murderer’s identity may have been revealed in the final moments of the first episode.
In a flashback to the night that Rachel (Anna Chancellor) was killed, housemaid Kirsten Lindstrom (Morven Christie) could be seen screaming as she discovered the body.
As she began to cry hysterically, Rachel’s husband Leo (Bill Nighy) held onto her on the stairway.
Meanwhile, the commotion attracted the attention of their other adoptive children Mary Durrant (Eleanor Tomlinson) and Hester (Ella Purnell).
Mary was dressed in an elaborately decorated dress as she fell to the floor in shock.
But Hester appeared in a white night gown, which featured a large blood stain on the front.
Fans may have missed the incriminating detail as the dramatic events unfolded – does it prove Hester is the culprit?
Fans of the Agatha Christie novel Ordeal By Innocence will know the identity of the killer.
However, it was recently revealed that writer of the BBC three-part drama, Sarah Phelps, had decided to change the ending.
She explained: “To the horror of Agatha Christie devotees I have changed the ending.
“I don’t give a b*******s about people saying it had to be pure.”
It comes after viewers were left reeling that they would have to wait a week until the second instalment.
Fans took to Twitter to express their dissatisfaction with the wait for the next episode.
“Why wasn’t Ordeal by Innocence dumped in iPlayer in one go?” one asked.
Another added: “Just watched Ordeal by Innocence and loved it. Shame the next 2 episodes aren’t on iPlayer now. Would have been up all night to finish it off.”
A third continued: “Should have shown it in one go #OrdealByInnocence.”
Ordeal By Innocence airs tonight on BBC One at 9pm.
