（话）还得从…说起
to explain this we must go back to …; it all starts with …
What it means
A storyteller's way into an explanation: the answer lies further back, in an origin, an event or a person. 'To explain this, we must go back to …; it all starts with …'.
How it is built
(要说 + question，) + 话／这 + 还得 + 从 + starting point + 说起: 要说这家书店为什么叫“半亩”，还得从老板的爷爷说起 (To explain the name, we have to go back to the owner's grandfather).
When to use it
Feature writing, talks and storytelling. 还得 ('will still have to') makes the detour sound necessary; 讲起 is a variant of 说起. Unlike 说起来 ('come to think of it') or the opener 话说…, it promises a backstory.
Watch out
得 is read děi here, not dé. After 从 put a starting point (a person, an event, a time) and close with 说起. Do not mix it up with 言归正传, which brings a speaker back to the main topic, not to the origin.