Complex sentence: The Next Doctor begins to regain some of his lost memories; when the Tenth Doctor finds a pair of "infostamps" (the Cybermen's data storage devices) the Next Doctor remembers he was holding one the night he lost his memory.
Split into simpler sentences: The Next Doctor begins to regain some of his lost memories. When the Tenth Doctor finds a pair of "infostamps" (the Cybermen's data storage devices), the Next Doctor remembers he was holding one the night he lost his memory.