Complex sentence: The Doctor discovers that the androids plan to open a time window to Reinette's life at the age of 37, believing that her brain at that age will be compatible with the ship's systems.
Split into simpler sentences: The Doctor discovers that the androids plan to open a time window to Reinette's life at the age of 37. The androids believe Reinette's brain at that age will be compatible with the ship's systems.