To use a Red Giant PluralEyes 3.5 serial number today, you must ensure you have the matching legacy installer and are using a compatible operating system, as this version entered a "limited maintenance" phase in 2023.  The "Solid Story" on Your Serial Number 

Serial Number Format: Legacy Red Giant serials typically consist of a four-letter prefix (identifying the product) followed by 16 digits.

If you are trying to get your old project synced or reinstalling a classic version of the software, here is everything you need to know about the current state of PluralEyes 3.5, how licensing works now, and what to do if you've lost your key. The History of PluralEyes 3.5

The glow of the dual monitors reflected in Elias’s glasses as he stared at the red "Trial Expired" banner cutting across his timeline. It was 3:00 AM, and the deadline for the indie documentary was eight hours away.