When Snap goes public next month, one of the biggest winners will be Lightspeed Venture Partners.

Lightspeed, a Silicon Valley venture capital firm, was the first institution to invest in Snap, the company that popularized disappearing messages, and it is now set to reap more than $1 billion from what began as a mere $485,000 investment.

But the big money for Lightspeed masks a complicated tale between the venture firm and Snap, the parent company of Snapchat. It is a story that offers a peek into the often opaque world of venture capital, into how start-ups begin and into the politics over money that accompany the relationships between companies and entrepreneurs. It is not a story that many of those involved want to discuss, especially with Snap executives now on a heavily hyped investor roadshow ahead of the public offering.

One of the biggest questions that Snap has faced from potential investors is why its two founders, Evan Spiegel and Bobby Murphy, have retained such a hold on voting power in the company — power that public shareholders will not gain. Exploring that question helps explain how years-ago dealings with venture capitalists helped lead to this point.

At the heart of that is a Lightspeed venture capitalist, Jeremy Liew, and the terms he embedded in his 2012 investment in what was then known as Snapchat. The terms gave Mr. Liew outsize power over the company’s future financing round. That ended up irking Snapchat’s chief executive, Mr. Spiegel, who took steps to reassert control over the company.